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?

[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?

[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.

[Clive]

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.