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Old 6th October 2012, 11:20 AM   #11
rmgvs is offline rmgvs  Netherlands
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- value pot: 50k or 100k log.
- resistor: see attachment, the 100pf is the small blue cap, I think it is the resistor just below the blue cap, check the value, must be around 10k, try putting another 10k over this resistor to get 5k new value.
- alternative: you could use a voltage divider just after the pot, but is more complicated
- MKP: you could try that, there is plenty of room inside the case, I would keep the value the same at 10uF. But not a problem to go as low as 1uF. It will affect bass response below 20hz (depends on load of power amp) but improbable that you could hear that. Just try.
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Old 6th October 2012, 08:57 PM   #12
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Thats brilliant advice - thanks a lot. I have to ask, how come you know this strange preamp so well ?
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Old 7th October 2012, 09:52 AM   #13
rmgvs is offline rmgvs  Netherlands
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I was curious about the good sound and the hiss. I tried to rebuilt the circuit by myself to try out different power supply arrangements and different mosfets like bs170. In this way you can learn a lot while not destroying the original.
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Old 11th November 2012, 12:02 AM   #14
ColinF is offline ColinF  New Zealand
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Hi,
I just had one of these preamps in for repair and decided to trace the circuit myself. Here it is! Hope you can read the backwards layout scribbled onto my notepad.
Colin.
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Old 11th November 2012, 12:06 AM   #15
ColinF is offline ColinF  New Zealand
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Phono stage:
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Old 11th November 2012, 12:07 AM   #16
ColinF is offline ColinF  New Zealand
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Power supply:
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Old 20th November 2012, 09:11 AM   #17
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Hi Colin... thanks for posting these drawings. I'm curious to know how you might implement a simple balance control on the pre-amp. I'm suffering imbalance (only slight mind you)

Also, regarding the main output cap (10mf).... what issues do you see in replacing with (say) 5mf ? Any improvement in the bass ?

What type do you recommend ?

Rgds
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Old 20th November 2012, 10:48 PM   #18
ColinF is offline ColinF  New Zealand
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Hi Steve,
The 10uf output cap was probably made to be such a large value because it is a cheap electrolytic, with attendant higher distortion. Depending on the input impedance of your power amp you could use a good quality film 1uf cap. With say 100k poweramp input impedance the bass roll off would still only be 2.3 hz. For the balance control, you could use an alps balance pot and put it electrically after the volume pot, in series with the 39 ohm resistor.
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Old 11th March 2013, 03:00 PM   #19
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Hi Colin

Is that output capacitor a reversible electrolytic or is it polarised ? (it shows on your diagram as polarised)

In fact, does it matter ?

Thanks

Steve
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Old 11th March 2013, 03:28 PM   #20
rmgvs is offline rmgvs  Netherlands
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It's a standard cheap polarised one. You can try anything between 1uF and 10mF as you like. I've tried a lot including boutique caps but am back now to the standard cap. Just listen and choose as you please. In the old days it was a sin to listen to cheap electrolytics but to be honest I like them a lot, it gives the music a flowing and easy-going quality (good be my ears/age, could be that the quality of electrolytics has improved a lot, I don't know).
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