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
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
Attachments
bypassing R13, 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
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
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
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
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.
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.
Correct
Dear Frank,
I think you are correct. Sorry about the mistake.
Is C18 68000pf? Looks like it to me. If so, it should be ok.
R15 should be increased, will help the bass response and add "flesh" to the voices. Might first try a large value, say 220k and see if it is ok or is too full. If to full, then back off the value till it satisfies your tastes.
Thanks for catching my mistake Frank.
Dear Frank,
I think you are correct. Sorry about the mistake.
Is C18 68000pf? Looks like it to me. If so, it should be ok.
R15 should be increased, will help the bass response and add "flesh" to the voices. Might first try a large value, say 220k and see if it is ok or is too full. If to full, then back off the value till it satisfies your tastes.
Thanks for catching my mistake Frank.
Hi,
68nanoF/630V it is.
Agreed, it's at least worth a try to put a 100K load there.
5K6 is really pulling the lot down heavily.
No problem, most everyone else overlooked it too.
The thing with these kits is that you can only change so much with these flaming PCBs.All along risking to overheat the traces as well...hate them things.
Long live P2P.
Cheers,
68nanoF/630V it is.
Agreed, it's at least worth a try to put a 100K load there.
5K6 is really pulling the lot down heavily.
No problem, most everyone else overlooked it too.
The thing with these kits is that you can only change so much with these flaming PCBs.All along risking to overheat the traces as well...hate them things.
Long live P2P.
Cheers,
C18
Hello!
You could even decrease C18 to 47nF or even lower and still get good bass response because at that junction (C18 and R10) you're getting a little positive feedback. I think like a bootstrap. So actually R10 (1 meg) appears larger than it is.
So as Frank said 68n (0.068uF) is plenty big.
And the top of R16 and R10 are not connected.
Wayne
Hello!
You could even decrease C18 to 47nF or even lower and still get good bass response because at that junction (C18 and R10) you're getting a little positive feedback. I think like a bootstrap. So actually R10 (1 meg) appears larger than it is.
So as Frank said 68n (0.068uF) is plenty big.
And the top of R16 and R10 are not connected.
Wayne
Wow!
Thanks for all the replies Guys! Sorry about the mis-type in the heading and the poor scan..
Some thoughts… The consensus seems to be that the coupling caps are ok into the impedances presented to them, thanks I did not factor this, I was looking at the caps in isolation.
That output load resistor does look small, think I will increase it significantly
I think I will also play with the PSU, some soft recovery diodes and replacing resistors with chokes ect.
I take the point about the cheap kit, you are right! I was attracted to it because it has some features that I like, relay input switching under micro processor control ect.
Is it possible the micro controller oscillator is affecting the sound? Should I try and shield it?
I thing scrapping the input coupling capacitor is a good idea, none of my sources seem to have a significant DC offset anyway.
The PCB is a pain, as is the limited space inside the case. This is really a last ditch attempt to salvage the unit before trying something else! P to P wiring of course!
Thanks again for the help!
Jim
Thanks for all the replies Guys! Sorry about the mis-type in the heading and the poor scan..
Some thoughts… The consensus seems to be that the coupling caps are ok into the impedances presented to them, thanks I did not factor this, I was looking at the caps in isolation.
That output load resistor does look small, think I will increase it significantly
I think I will also play with the PSU, some soft recovery diodes and replacing resistors with chokes ect.
I take the point about the cheap kit, you are right! I was attracted to it because it has some features that I like, relay input switching under micro processor control ect.
Is it possible the micro controller oscillator is affecting the sound? Should I try and shield it?
I thing scrapping the input coupling capacitor is a good idea, none of my sources seem to have a significant DC offset anyway.
The PCB is a pain, as is the limited space inside the case. This is really a last ditch attempt to salvage the unit before trying something else! P to P wiring of course!
Thanks again for the help!
Jim
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