Tweaking an 80s preamp

**changer** · 26-11-2020, 10:15

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/transla...ead-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.

**changer** · 26-11-2020, 10:23

Originally Posted by Clive

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.

**Macca** · 26-11-2020, 11:56

Originally Posted by changer

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/transla...ead-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.

**changer** · 28-11-2020, 15:52

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.

**Macca** · 28-11-2020, 15:55

I think some of that might have been lost in translation...you are saying that there's an issue with the VU meters?

**Clive** · 28-11-2020, 16:02

Originally Posted by changer

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.

**changer** · 28-11-2020, 16:30

Originally Posted by Macca

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.

**changer** · 08-12-2020, 21:26

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?

**Macca** · 08-12-2020, 21:53

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.

**changer** · 09-12-2020, 17:08

Originally Posted by Macca

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.

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