Tweaking an 80s preamp

[Clive]

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.

[Mr.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.

Thanks Clive

[changer]

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 ...
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.
IMG_1515.jpg
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.

IMG_1516.jpg
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.

IMG_1517.jpg
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?

[Clive]

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.

[changer]

Thank you very much and it will be nice to see, how you did.

[Clive]

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.

[changer]

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.
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How do you actually like the sound of this combination?

[Clive]

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.

[changer]

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.

[changer]

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?