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Old 23rd June 2008, 03:43 PM   #1
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Default Tube phono amp repair/redesign help wanted

Hi,

I recently received an custom built (DIY) all-tube pre-amp. The line-stage works just GREAT but the phono-stage is not so good at all. It sounds to low, no dynamics and as if the tweeter was not working at all. Something is wrong and I suspect the design of the schematics to be off (wrong Cap/resistor vlaues probably?). Problem is, the person who built it cannot hear it as I do and could not fix it either.
Would anybody here be willing to run the schematics (which I have in PDF format) through spice or tubecad or whatever tool available, to check if all the parts values calculate correctly? This would be of tremendous help to me.

The schematics is very similar to the pre-amp described in the book "hoechst empfindlich" of Goetz Wilimzig.

2 E88CC in the phono-stage and 1 E280F (Triode mode) in the line-stage, with passive RIAA EQ.

I am an illiterate when it comes to electronics-design but like to build things. So I could try to fix it once somebody who knows better has determined the correct values or found a theorethic error.

Many thanks in advance for any help offered!

Dieter
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Old 23rd June 2008, 03:56 PM   #2
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Let me ask the obvious question: what is your cartridge and what is the loading impedance of the preamp? If you load a MM cart with a very low resistance suitable for MC you may experience some very dull sound. Of course it's best to post the circuit.
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Old 23rd June 2008, 04:17 PM   #3
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Hi,
It is not a cartridge impedance problem. I tried various MM & MC/Step-Up set-up and compared the results to other (OP-Amp & Tube) phono stages.
This baby always came out way last.

But for the record: I tried a Shure MM with 90K load & 47K load. I tried various capacities with NO impact on the sound quality of this Pre.
Same (bad) results with a clearaudio Stradivari (MC) via a Silvercore 1:10 100K Step-Up which usually sounds marvelous, using 47K, 90K and NO load at the phono-input.

cheers, Dieter
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Old 23rd June 2008, 04:30 PM   #4
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Quote:
Originally posted by dikarner
NO load at the phono-input.


Hi Dieter

Sorry for the trivial question, i didn't realise you were a seasoned vinyl-phile

A circuit seems essential then. Any idea what "NO load" means? There is obviously a resistor at the first tube's grid which determines the maximum value of the input resistance. I assume you change resistors in parallel to this internal resistor to set your desired value. The difference between 90k and 47k is usually quite subtle; what i had in mind was loading a MM cart (or the secondary of a transformer) with a 100ohms.
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Old 23rd June 2008, 11:41 PM   #5
Sheldon is offline Sheldon  United States
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Default Re: Tube phono amp repair/redesign help wanted

Quote:
Originally posted by dikarner
Many thanks in advance for any help offered!
You'll get lots of help, but you need to provide two pieces of info: Schematic, and actual voltages at each tubes plate and cathode, as a minimum.

Sheldon
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Old 24th June 2008, 05:22 AM   #6
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Default Schematics

I just now realized I could upload as well . I hope this will suffice for anybody willing to do the math on the RIAA inverson:

There's one small error: the last resistor, connecting the Output to ground (2,2Meg) is missing on the circuit diagram.

The values are as published in the book. Actual, if different, values are provided in brackets. Major differences to the official version are marked in red.

The descriptions are in German:
müF --> uF
U1/U2 --> V1/V2
fehlt im Original --> does not exist in original schematics

BR
Dieter
Attached Files
File Type: pdf tmt vorstufe-schaltung phonoteil.pdf (58.9 KB, 228 views)
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Old 24th June 2008, 06:37 AM   #7
oshifis is offline oshifis  Hungary
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The capacitors are 10x as large than they should be.
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Old 24th June 2008, 07:25 AM   #8
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... there's another issue which I did not mention earlier:
I'd be very happy if next to the filtering the gain could be improved as well. I think two 6922 should be able to amplify a MM Cartridge to the same level like a Tuner or CD Player, which is not the case currently.

D
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Old 24th June 2008, 07:29 AM   #9
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@oshifis

not meaning to be rude , but could you please be more precise?

thx
Dieter
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Old 24th June 2008, 08:23 AM   #10
oshifis is offline oshifis  Hungary
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Sorry, I wrote it in a hurry. I refer to the paper of Stanley P. Lipschitz titled "On RIAA Equalization Networks" (J.A.E.S. 1979 June). See section 8 Addendum where he describes the formula for passive eq networks. The relevant formulas are:

R1 * C1 = 2187 us
R2 * C2 = 109.05 us
R1 * C2 = 750 us
R2 * C1 = 318 us
R1 / R2 = 6.877
C1 / C2 = 2.916

R2 is R3 on your diagram, C1 is C4 and C2 is C5.

C4 = 3.0 nF and C5 = 1.0 nF give quite good values. The plate resistance ( cca. 16k according to my calculation) adds to R1.
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