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Old 13th March 2004, 02:58 PM   #1
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Default Help needed with pre-map.

Dear all,

Attched below in the circuit of my velleman kit pre amplifier.
The specs of this amp seem fine, but i have never been happy with the sound. It sounds bright and washed out.
Being new to tubes, i would like some sugestions as to what i can do to improve the design. The things that seem odd to me are:-
1. The three coupling capacitors vary greatly in value, with the coupling capacitor between the stages being very small in my opinion (C19?)
2. I undertand the first stage to be a simple anode follower, however why is the second stage cathode coupled, why is it there at all,as i believe the first stage provides the gain, is it meant to reduce output imedance?
3. how is feedback applied.


I hope you can help!

Jim
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Old 13th March 2004, 11:55 PM   #2
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Hi,

Quote:
1. The three coupling capacitors vary greatly in value, with the coupling capacitor between the stages being very small in my opinion (C19?)
Yes, but so do the input impedances so those values seem more than adequate.

Quote:
is it meant to reduce output imedance?
Yes.

Quote:
3. how is feedback applied.
The last stage has 100% degenerative FB.

I suppose that relay and transistor vary the overall gain by 6 dB by shortcircuiting the bottom cathode resistor of the first stage.

Cheers,
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Old 14th March 2004, 04:49 AM   #3
SHiFTY is offline SHiFTY  New Zealand
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You could try bypassing the cathode resistors in the first stage, try putting a 100uF 25V cap across R13 and another across R17...

Also C18 (the 68n 630V) could be increased a bit, try doubling it and see how you go.
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Old 14th March 2004, 04:22 PM   #4
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Default bypassing R13, R17

Quote:
You could try bypassing the cathode resistors in the first stage, try putting a 100uF 25V cap across R13 and another across R17...
ShiFtY:

Wouldn't you loose the function of the +6db switch all together if you bypass both at the same time? If you were to go that route wouldn't it be better to drop R17 and the relay altogether?

IMHO I would change R15 to 56k or 100k and C18 to 100n or leave as is (68n). 5.6k seems like an awfully low load for a cathode follower! I will do simulation of the circuit and see what I come up with.

Wayne
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Old 14th March 2004, 07:04 PM   #5
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Hi,

The 68nF cap couples into a 10M impedance, surely that's big enough.

The 5K6 bleeder is a load resistor forcing a constant current through the circuit.

Why it sounds as described I don't know, the time constants through the RC networks seem fine...

Cheers,
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Old 14th March 2004, 08:36 PM   #6
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Default 5k6 bleeder

Hi Frank

If you've noticed, that 5k6 resistor is after the output coupling capacitor. You wouldn't use this preamp to drive a Sand Amp with a 5k input imp. ? It's the just the same. Distortion will rise, sound would be tinny and output voltage would be limited. I would'nt use this preamp to drive any load lower than 20k at best let alone 5 kilo ohm.
I did quite a few sims and with that 5k6 bleeder resistor:

Output @ 1kHz = 8.7db, down 3db @5.4 Hz.

With 100k or 56k bleeder:
Output @1 kHz 10.3db, down 3db @ 0.5 Hz.
Also much less phase shift at lower freq. as expected.

I have never seen such a low output cap bleeder. I've seen this with IC opamps and some SS circuitry.
Maybe with a White cathode follower you could drive a 10k load.
Correct me if I'm wrong, but I would change that resistor!

Cheers

Wayne
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Old 14th March 2004, 09:11 PM   #7
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Quote:
It sounds bright and washed out.
The topology is probably not to blame. Washed out sounds pretty much like an ECC82. Add a PSU with slow diodes and bad-sounding caps and i see why you don't like it. How about a complete rebuilt with a different tube (5687?) and some PSU upgrades?
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Old 14th March 2004, 11:24 PM   #8
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Hi James,

Looking at C18 (and R16, 10, 11), no wonder it sounds thin. Should be much larger if you want descent lows and flesh on the "faces". The other two caps also form high pass filters too.

Increasing C18 will help lows but be careful of possible motorboating from weak power supply design. If this happens, will have to lessen value of C18. Would try to give it 6db leeway in this regard.

Also might try getting rid of C17.

If I remember correctly, Velleman is rather inexpensive, so you may not be able to do alot without major modifications.
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Old 14th March 2004, 11:37 PM   #9
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Default R15

R15 is even worse, 5.6k?! Increase it alot, even to 220k or more if desired without harm, and see how it sounds. Shouldn't be any motorboating problems as this resistor is "out" of the circuit design that could cause motorboating.

Touch up C18 later, if you need to.
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Old 15th March 2004, 01:24 AM   #10
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Hi,

Quote:
Looking at C18 (and R16, 10, 11), no wonder it sounds thin. Should be much larger if you want descent lows and flesh on the "faces". The other two caps also form high pass filters too.
Sorry, but I don't think so.
The scan is bad but as far as I can tell the top of R10 and R16 is not connected together. Why would they be anyway?

So C18 is seeing R10 + R11. That's over 10M.

Cheers,
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