View Full Version : Tweaking an 80s preamp
I have a Phillips Black Tulip 22AH280 preamp. It's quite a sophisticated circuit, not typical Phillips of the era - infact I believe it was designed and built in Japan. I've looked at the circuit, I don't want to make any major changes but some capacitors leap out at me as being candidates for improving:
Phono stage output: a series pair of 10uF electrolytics making them bipolar. Replace with one 4.7uF polyprop.
Linestage input: there's an NPN transistor fed by a 470nF electrolytic and a PNP fed by another 470uF electrolytic with the capacitor in reversed polarity. Could I replace these capacitors with polyprops or as I suspect is polarity important here? I'm confused by this configuration.
Linestage output: a series pair of 10uF electrolytics making them bipolar. Replace with one 4.7uF polyprop.
The service manual with the circuit is at:
https://www.vintageshifi.com/repert....php?pdf=Philips-22-AH-280-Service-Manual.pdf
I'm thinking the series capacitors in particular could be changed, I suppose suitable film capacitors weren't available in the 80s or were very expensive. Is this wise?
https://oi43.photobucket.com/albums/e359/cmeakins/280_LI_zpsshak5lp4.jpg (https://s43.photobucket.com/user/cmeakins/media/280_LI_zpsshak5lp4.jpg.html)
Firebottle
17-04-2019, 10:17
Very wise, yes I would replace all with PP.
Very wise, yes I would replace all with PP.
Hi Alan, thanks...can you shed any light on the two 470nF caps at the input side ie why the are in opposing polarities? I understand why the 10uF caps are configured as they are but the 470nF ones mystify me - that's not a difficult thing to do! I'm concerned that polarity matters at the input side for some reason I don't understand - so would PP caps be ok in that position?
Firebottle
17-04-2019, 10:55
Yes it's a split supply buffer stage, so the small bias voltage on the transistors is going to be positive on the top one and negative on the bottom one, with respect to ground.
Another good reason to replace with PPs, electrolytics aren't best when they only have a small polarising voltage.
I get it now. Thanks, I'm now confident non-polar caps will be fine in that position. I didn't want to get it wrong!
I swapped the electrolytics in the signal path for PP and also what was probably a polyester type for PP too. The change in sound is dramatic, how much is down to better a far type of capacitor and how much is down the electrolytics being 40 years old is hard to know...but it's an awful thing to have electolytics in the signal path...3 sets if I include the phono stage. The preamp is sounding darned good now.
Firebottle
22-04-2019, 20:11
Excellent.
Just bought one of these !! came up at a price I couldn't resist. Any other upgrade suggestions. Do you know what the switched outputs are - for MFB speakers ?
Just bought one of these !! came up at a price I couldn't resist. Any other upgrade suggestions. Do you know what the switched outputs are - for MFB speakers ?
The circuit diagram suggests the switched output is for headphones - in addition to the main front mounted headphone socket.
On the rear there's the main 2V output, the switched 2V output (phones) and a 3.6V output / +10db output.
Certainly swap out any polarised caps in the signal path. The improvement was significant. Mine is / was a European version and was wired for 220V, it's a bit fiddly to get to the mains wiring but it's a good idea to switch it to the 240V tap on the transformer, if you have the same setup.
Hi Clive what make of caps did you use in the end anything special?
Ian
Hi Ian, I used clarity CSA for area... I used some other polyprops for the smaller value ones, getting some to git6 is the challenge for the smaller ones. Just go for low cost but check sizing.
Hi Ian, I used clarity CSA for area... I used some other polyprops for the smaller value ones, getting some to git6 is the challenge for the smaller ones. Just go for low cost but check sizing.
Thanks Clive
Hello Clive!
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I registered here to ask you a for a favor: Would you mind to write some corrections and maybe add some details to my reconstruction of your improvements on this pre-amp? I will not do it too soon and there would not be no hustle. If you find some time and leisure, I would be thankful. I have a technical understanding and I can solder, but I have never dealt too much with (audio) electronics and do not want to short circuit something or misconnect. In my first attempt to localize every component on the circuit plan, I already marked a resistor instead of a cap ...:scratch:
That is why I want to make myself a detailed set of instructions before I begin with any tweak.
Phono stage output: a series pair of 10uF electrolytics making them bipolar. Replace with one 4.7uF polyprop.
I think it is these four fellows here: C502a/b, C503a/b, each 10µF.
28868
How will I replace with one 4,7µF? Will I connect the replacement capacitor with one pole at the soldering point of C502a/b and the other at C503a/b? In which way do I have solder the poles?
Before, these are like ---||+----+||--- (4 poles, two each for 503 and 502)
After, will it look like this, the other way round or another way?: ---|[skippend |+]---+|[skipped|]--- (2 poles, one from 503 and one from 502)
Same question goes for the Line output, but it must be the same principle I suppose.
Linestage input: there's an NPN transistor fed by a 470nF electrolytic and a PNP fed by another 470uF electrolytic with the capacitor in reversed polarity. Could I replace these capacitors with polyprops or as I suspect is polarity important here? I'm confused by this configuration.
28869
I was not too sure, so I wanted to ask: You replaced both 516a/b and 517a/b, am I right? The black and the lilac 4,7µF caps, marked in two different colors on my picture.
The replacement Allan suggested of C519a/b is clear I think.
Linestage output: a series pair of 10uF electrolytics making them bipolar. Replace with one 4.7uF polyprop.
28870
The same question on polarity as with Phono stage output above: Which poles of the soldering point from C918a/b and C919a/b should be connected?
Hi Marinus,
That all looks correct. To be 100% sure I’d need to lift the lid on my preamp. I’ll try to do this next week and take photos.
The 4 capacitors at the side of the phono stage were a pain as I couldn’t find small ones so fitting them was a bit messy.
The pairs of electrolytics are replaced with non-polar polypropylenes so there is no polarity to worry about. Just solder to the “outer” connections, which I think are are negative ones.
Thank you very much and it will be nice to see, how you did.
Here are the pics of the changes I made. For the 4 large capacitors on the phono stage board I needed to fit pcb pins as the capacitor wires were too thick for the holes in the pcb. If you can find smaller capacitors it’ll make your life easier.
https://i.postimg.cc/G3SxGRf3/Pic-1.jpg
https://i.postimg.cc/yxWhtpDz/Pic-2.jpg
https://i.postimg.cc/wMP5p5NR/Pic-3.jpg
https://i.postimg.cc/BQh4xZR2/BT.jpg
Thank you for the illustrative images Clive. I may come back with a question or two, when I order the parts. But I think it is all rather self explanatory.
By the way, do you use preamp and poweramp on top of one another or was it just to show them here? In a test from 1980, they suggested to avoid this: The potentiometer of the preamp would otherwise introduce a hum pickup.
2889128890288892888828887
How do you actually like the sound of this combination?
I don’t get any hum though when I’ve used the amps with my 100db/m speakers I had the pre and power separated. With the simple capacitor upgrades I found the preamp came up to the standard of the power amp. I’d be happy with the sound of this combination in my main system....though I confess it’s currently in a secondary system.
I wish my German were better than being just enough for ordering beer and asking for receipts. The review must be interesting.
Cheers! Sad thing is you could not even practice basic German right now, because all the bars are closed :( Here is what the algorithm translated from the conclusion, the translation is okay, I checked (Page 5). The data they are referring to is found in the columns of the 3rd and especially the 4th image I uploaded (very bottom right), with the faulted signal-to-noise ratio of the combination (!, not the preamp):
Philips 22 AH 280 SA / 380 PA
The preamplifier 280 SA delivers a very high level at output 3 with constant internal resistance. However, with regard to the signal-to-noise ratio, it is better to use outputs 1 and/or 2. Frequency range, frequency response and phono equalisation are OK. The characteristic curves of the filters are also to be praised. The input sensitivities are, taken by themselves, ok, those of the combination even better. The input sensitivity of the microphone input is 1.7 mV-55.5 dBV at a maximum input level of 180 mV a - 15 dBV, which gives an overload reserve of 40.5 dB. The signal-to-noise ratio of the preamplifier alone is excellent, that of the combination, operated via output 1, only mediocre when referred to 2 x 50 mW output power. The two Philips components should also not be placed on top of each other, because the apparently quite high-impedance potentiometer of the volume control is sensitive to hum from the power supply of the power amplifier.
The power amplifier 380 PA has a wide frequency range and a power bandwidth of more than 100 kHz, which suggests a "fast" output stage! Dynamic distortions (TIM) are therefore not to be feared. The power-distortion diagram (Fig. 2.5) gives the combination quite a good report.
In the music listening test the first thing to be examined was the practical influence of the only moderate monitor cross-talk attenuation. This value depends on the terminating resistance of the connected programme sources and can be disturbing under adverse conditions when very high quality crosstalk on reel-to-reel tape recorders is involved. The problem is not relevant in connection with a cassette recorder.
Opulent HiFi volume is achieved on the Sentry III in position 4 of the level control on the power amplifier and in position -7 of the volume control on the pre-amplifier. If the Revox B 530 is connected, positions 5 (on the power amplifier) and -7 (on the pre-amplifier) must be selected for the same sound level. In these positions of the controls, there is no hum and only some noise on the phono when the tone arm is raised. If everything is turned up to full power, relatively strong noise and a distinct hum can be heard. By the way, it is very advisable to turn the level controls on the power amplifier back far enough so that you can work with the volume control of the preamplifier in the upper third, because the gradations with 1 dB steps are sufficiently fine there and you then have an optimal effect of the well designed physiological volume correction even in lower level ranges.
The sound of the Philips combination remains very clean, transparent and impulse resistant up to high volumes, provided that the speakers are able to keep up. The power reserves are considerable when using loudspeakers with not too high practical operating power. The power amplifier's display instruments are very praiseworthy, showing really peak values, with forward and reverse damping of the pointer being practical.
Two weaker points are the moderate crosstalk attenuation of the pre-band on rear-band playback and the rear-band on recording, which, however, can only lead to interference in connection with high-quality reel-to-reel tape recorders and the highest demands, and the only average signal-to-noise ratio. based on 2 x 50 mW. When installing in racks, it is advisable not to place the pre-amplifier and power amplifier directly above each other. Otherwise the signal-external voltage distance will deteriorate in both combinations due to hum interference.
One more question:
Just go for low cost but check sizing.
What is acceptable "low cost" for you in this case? Perhaps I am more on a budget than others and I could misunderstand. Eight CSA 4,7mF caps cost me about 150 euro, which is almost the preamp's price. I would like to go cheaper but only if it is acceptable. Any suggetions?
Also, could it be that I made a mistake? 470nF is 0,47µF. Accordingly, it would be 0,47µF capacitors at linestage input, and 4,7µF capacitors ib phonostage input and linestage output?
One more question:
Just go for low cost but check sizing.
What is acceptable "low cost" for you in this case? Perhaps I am more on a budget than others and I could misunderstand. Eight CSA 4,7mF caps cost me about 150 euro, which is almost the preamp's price. I would like to go cheaper but only if it is acceptable. Any suggetions?
Also, could it be that I made a mistake: 470nF is 0,47µF. Accordingly, it would be 0,47µF capacitors at linestage input, and 4,7µF capacitors ib phonostage input and linestage output.
First of all, thank you for the translation on the conclusions..very interesting.
My post about capacitor types at the top of page 2 is full of typos...
I used a mix of JB Capacitors - JFX series and Clarity Cap SA series. The large phono stage ones are 0,47µF (I think they are from the poor photo...) - in the UK they are £3.20 each. The JFX 4,7µF are £2 each.
There's no magic with my choices. You could for example use Solen polypropylene capacitors. Also film capacitor will be much better than the originals.
I've checked the order I sent to HiFi Collective.
4 x 0,47µF Clarity Cap SA
4 x 4,7µF JP Capacitor JFX
4 x 1µF JP Capacitor JFX
There must be 2 x 1µF which I didn't take photos of.
Maybe two of the 1µF were for a different project, I really don't recall. Sorry...
Thanks for the clarification. According to Humble Homemade Hifi, the MKP caps you chose should be close to the original tonal balance of the replaced electrolytics:
"In some cases the 'flat' sound can work well, for example if you just want to keep things tame or if you are replacing some old electrolytics or MKT's and don't want to shift the tonal balance too much."
http://www.humblehomemadehifi.com/Cap.html
What do you think about the signal-to-noise ratio of the two amps playing together?
https://i.postimg.cc/0MWWJJj4/20200519-190218.jpg (https://postimg.cc/0MWWJJj4)
Section C, section B is power amp only.
https://i.postimg.cc/cg4kHcxY/20200519-190212.jpg (https://postimg.cc/cg4kHcxY)
The preamp, right column only.
I could buy the poweramp, I only have the preamp now. But the one I found at a good price has a yellowish shimmer in the interior. Is it from smoking or normal aging? Should I stay away from it? The outside looks much better, pretty good actually.
https://i.postimg.cc/bSJ9Z0ZD/m-konversation-anhang-html.jpg (https://postimg.cc/bSJ9Z0ZD)
https://i.postimg.cc/G9hQgRXL/m-konversation-anhang-html2.jpg (https://postimg.cc/G9hQgRXL)
The terminals were exchanged, this is why we see non-original cables in the top section.
The yellow might just be the lighting changing the white balance of the photo. Depends on the price...
The owner said it is not only the whitebalance, he put the paper for comparison. I can have a look myself on the weekend.
Negotiations before the interior images where shared: 275€ (245£)
That's a cheap price, I paid £350 for mine. It's a good power amp, easily worth the money.
Yes the price is good. A lot of the yellowing looks to be on the internal speaker cables. If these were not yellow the overall effect would be much more normal. Cables can age like that or maybe they had some colour originally.
Re the noise figures...all I can say is that I have no noise issues when the pre/power is plugged into my 100db sensitivity speakers.
Good morning Martin,
thank you for your opinion. Nice to hear you own this amp, it is a beauty and should suffice my needs aswell. The price is good, yes, given the equipment holds and will not take damage from this surface coat. Is it nicotine? I got to go check it out.
Another question: Whoever replaced the terminals was quite generous with the output cabling of the speakers. Should I make them shorter again, so their interference is reduced? In a test on a German board, they found that the left channel is picking up noise from the grid because the input power line is internally too close to the left channels input stage (https://translate.google.com/translate?sl=de&tl=en&u=https://old-fidelity.de/thread-38929.html). Not too dramatic but could be improved. It is probably a different magnitude I suppose, but maybe the speaker cables also radiate if they are as close to the amplifying electronics as seen on the picture? At least the productiong engineers held them rather short. My understanding of electronics is a bit limited, that is why I ask.
Re the noise figures...all I can say is that I have no noise issues when the pre/power is plugged into my 100db sensitivity speakers.
Hey Clive, that is also a good note from experience! I tried to find out the practical implication of a measurement of SNR at 2 x 50mW / 4 Ohm, and some tests still bring this measuring up, but I would not know what it actually means in listening.
Good morning Martin,
thank you for your opinion. Nice to hear you own this amp, it is a beauty and should suffice my needs aswell. The price is good, yes, given the equipment holds and will not take damage from this surface coat. Is it nicotine? I got to go check it out.
Another question: Whoever replaced the terminals was quite generous with the output cabling of the speakers. Should I make them shorter again, so their interference is reduced? In a test on a German board, they found that the left channel is picking up noise from the grid because the input power line is internally too close to the left channels input stage (https://translate.google.com/translate?sl=de&tl=en&u=https://old-fidelity.de/thread-38929.html). Not too dramatic but could be improved. It is probably a different magnitude I suppose, but maybe the speaker cables also radiate if they are as close to the amplifying electronics as seen on the picture? At least the productiong engineers held them rather short. My understanding of electronics is a bit limited, that is why I ask.
Hi Marinus
yes I have the pre-power combo, looking to collect the rest of the 'stack'.
Can't help you with the technical question though, it probably would make sense to have the wires as short as possible and route them optimally, I'd be very surprised if such a small difference was audible though.
The staining could be nicotine, then again some cable jackets do discolour over time anyway. If the amp stinks of smoke when you get it then you know! I use baby wipes to clean off nicotine staining, works effectively.
With your power amps, is there also this very small current on the front panel? Like one knows from old floor standing lamps that have this little extra friction, when you move your fingers over the metal and they do not glide, rather stutter? I had a look today and have to consider. I need to replace the light bulbs of the VU meters, which should not be a big deal, but this means extra work and time lost.
I think some of that might have been lost in translation...you are saying that there's an issue with the VU meters?
With your power amps, is there also this very small current on the front panel? Like one knows from old floor standing lamps that have this little extra friction, when you move your fingers over the metal and they do not glide, rather stutter? I had a look today and have to consider. I need to replace the light bulbs of the VU meters, which should not be a big deal, but this means extra work and time lost.
The cause of the “friction” is a lack of grounding. Both pre and power amps use 2-core mains cable. I’ve not checked but I think ground goes via the interconnects to my DAC. Grounding and mains earth is a system aspect to address. I’d rather have mains earth connected to every chassis but to do this without hum you need the signal totally separated from the chassis. I’ve not assessed this. Checking for continuity between the RCA ground and the chassis would be helpful.
I think some of that might have been lost in translation...you are saying that there's an issue with the VU meters?
Sorry Martin, I wrote about two different things without a proper break: One is that 3 of the 4 lightbulbs of the VU meters are not working, which is not bad because they can probably be replaced, but it is tedious work. Basically, I was whining.
The other thing is that I perceived a small current running on the front panel. I was wondering whether, as Clive put it, it is always there when the power amp is not grounded and grounding will solve this. Or, if it is so because somewhere there is a bridge to the enclosure which should not be there. I was not sure whether this is typical or if it is with this specific power amp and it was a reason for me to not take it with me this time. With my preamp I cannot sense the least bit of electrical current on the aluminium frontpanel, even if not grounded. So I was wondering if you can make out a difference when you have it switched on and when it is off.
How would you describe the difference of sound when the preamp is either connected to the DC input or the AC input of the power amp?
I thought the DC input offers the better sound: more clarity, resolution and detail, also the bass is more controlled. Then my GF said that the male voice of the singer has more character when powered with the normal inputs. So I listened and she is right in a way, the voice has more foundation and the lower mids tend to resonate. Is this the effect of the electrolytics in the 'normal' input stage of the power amp, a somewhat resonant amplification of the lower midrange? The overall sound of the Normal-input seems a bit granular too. What where your experiences and listening impressions?
I've nothing to measure current with but I don't get any sort of tingling sensation when touching the power amp that might indicate a bit of stray current. Mine does have a 3 pin UK plug though, things may be different on your electrical system.
I prefer the variable inputs, they seem a bit cleaner but I may be imagining that.
I prefer the variable inputs, they seem a bit cleaner but I may be imagining that.
This is normal, right? Interesting, I thought it was the other way around.
This is normal, right? Interesting, I thought it was the other way around.
a lot of these little differences are all in the mind. If there's no reason why it should sound different then it almost certainly doesn't.
Haha, my wording was imprecise, again. I meant: variable is (the) 'normal' (ac) input, right?
A lot of things in sound do not actually happen in the air, but in one's mind. This is precisely why I want to discriminate better what is purely subjective and what is actually happening in the amps. There is an audible difference between the DC inputs and the normal, so I thought maybe you two know something. Clive seemed to prefer the DC input (in another forum). By now, I still lack criteria for 'good': My speakers are only cheap early 90s Canton Karat 920 DC. I will get them replaced in a while. Until then, I can only do comparisions between those amps I have around and see how they differ: an entry level Rotel integrated (RA-921), and alternatively to the AH 380, an Behringer A800 PA power amp. Both are clearly outperformed by the Philips 380 when it comes to resolution, detail and especially midrange reproduction, which the Philips does really nice. The Rotel is just mud and distorts the low end. The Behringer power amp again has much more bass authority than the Philips, allthoug it was tested with only 150 watts per channel over at Audio Science Review. This makes it's output comparable to the AH 380. I do not know if this bass authority (it is so dry, controlled, direct and powerful!) is because Class D technology delivers energy more explosively in the low frequencies or whether I need to restore something in the Philips power amp? The Philips is great in the mids! This is still bugging me, I miss some deeper bass. The Rotel sounds 'bassy' in a bad way, but I have to use tone controls to get deep volume from the Philips. Is this actually a sign of quality: a flat amp, connected to little speakers, not generating deep volume?
In theory most amps should drive most speakers, in practice I have found differently, same as you.
Usually I use a Krell and this does have better bass performance than the AH 380 as you describe. The Philips just doesn't seem to have the same wallop. But it does give a sweet and clean sound which IME separates it from a lot of other solid state power amps.
The only mod I want to do on mine is sort out some better binding posts for the speaker cable. I was going to keep it original but they are just too awkward to use and won't take my preferred speaker cable.
The only mod I want to do on mine is sort out some better binding posts for the speaker cable. I was going to keep it original but they are just too awkward to use and won't take my preferred speaker cable.
The original terminals can be removed without damage (they have a 'latch'). You may keep them and reverse your update at any time.
Re DC vs AC inputs....it’s too long since I messed about with the amp in my best system. Differences with the speakers it’s currently hooked up to will be far less obvious...and I’ve not experimented with that setup so I can’t really give an opinion.
The original terminals can be removed without damage (they have a 'latch'). You may keep them and reverse your update at any time.
Have you changed yours and if so what did you use as a replacement?
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