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In Reply to: RE: Poor Design for a Preamp posted by Tre' on February 16, 2025 at 12:57:28
"You can't drive into a 43 ohm load to ground with a stage that has a 8.3k output impedance. . . . 8.3k output wants to see a 83k load."
OK, so in the case of this 01A preamp, he could use a voltage divider with 68k in series and 22k to ground. This would reduce the voltage about the same as the 330k/100k combo he's using and it would present a load of 90k to the tube and be no problem.
The value of 68k || 22k is 16.62k. If we plug that into the low pass calculator then the -3db point would be 54.75k. That should also not be an issue, right?
Follow Ups:
"OK, so in the case of this 01A preamp, he could use a voltage divider with 68k in series and 22k to ground. This would reduce the voltage about the same as the 330k/100k combo he's using and it would present a load of 90k to the tube and be no problem."Yes, you are getting this!
"The value of 68k || 22k is 16.62k. If we plug that into the low pass calculator then the -3db point would be 54.75k. That should also not be an issue, right?"
Well, almost. With a -3db point of 54.75kHz the -1db point will be 27.375kHz. That is out of the audio band but the phase is still being shifted by the filter. To get the filter completely out of the audio band the -3db point of the filter needs to be a full decade above 20kHZ.
***Also remember that these calculations do not take into account the Miller of the first stage of the power amp and that could be more capacitance than the cable.***
So to keep the filter totally out of the picture it needs a -3db point at 200kHz.
There are other things in a circuit that will not allow 200kHz bandwidth and there is not much we can do about it (200kHz would be almost impossible for an output transformer) but I don't see that as an excuse to not solve a problem that can be solved. These things stack up so there is no reason to exacerbate the problem.
My question would be, why does he need this gain reduction in the first place? Doesn't the preamp have a volume control in front of the tube stage?
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 02/16/25 02/16/25
I like your 22k//68K idea. Yet to put the preamp on the CRO. Gain was reduced because there was a hum issue.
| retro-thermionic |
Are you using AC for the 5 volt filament?
You might want to try a critical inductance input choke filter DC supply for a DHT.
Or maybe try Rod Coleman's Filament Regulators.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Well filtered 6V then a 5V regulator. Also balanced 100ohm/100ohm network with Rk central tap. Hum is not in the filament voltage.
| retro-thermionic |
I guess not. So it is either the B+ or a hum field.What happens when you move the power supply chassis as far away as possible?
Do you have a schematic of the power supply?
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 02/16/25 02/16/25
Finally got the preamp on the CRO. With the 300K/100K config, through the DIY shieldless cable (175pF), the upper 3db point was 19khZ into 100K. But, taking the 68K/22K suggestion lead to a much better upper 3db;- 37khZ. Not only that but the preamp was even quieter. One channel reading 0.0mV! Plaid all afternoon and throughly enjoyed it.Gain though is only 3db but these are power tubes so what do you expect? It will deliver 4Vpp into 100K. My DIY SE UL EL34 amp has plenty of gain so no concern. Not only that the two are a perfect synergy. Love the sound of this 6N7 preamp and love the metal tubes.
| retro-thermionic |
Edits: 02/26/25
"With the 300K/100K config, through the DIY shieldless cable (175pF), the upper 3db point was 19khZ into 100K. But, taking the 68K/22K suggestion lead to a much better upper 3db;- 37khZ. Not only that but the preamp was even quieter. One channel reading 0.0mV! Plaid all afternoon and throughly enjoyed it.
Gain though is only 3db but these are power tubes so what do you expect? It will deliver 4Vpp into 100K. My DIY SE UL EL34 amp has plenty of gain so no concern. Not only that the two are a perfect synergy. Love the sound of this 6N7 preamp and love the metal tubes."
6N7? Metal tubes? Huh?? Isn't this thread about a 01A preamp?
Looks like I got my threads mixed. But I've had the same problem over four different preamps based on the 01a, 71a, 6N7 and 12AX7 preamps. Forgive the mixup but the result (good result) hopefully will be the same.
| retro-thermionic |
It maybe an earth configuration issue. Ive used my design HT double Pi filtration in wellover 100 builds. Power amps and preamps. None have hums.
| retro-thermionic |
I see the preamp PS is earth grounded. Is the power amp earth grounded?
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
"I like your 22k//68K idea."Yeah, if you need to place the gain reduction solution on the preamp I figured that would be a good compromise.
On the other hand, if you could put it on the amp end of the interconnect that would seem to be even better.
If I'm understanding this correctly, that would lower the output impedance of the preamp and result in a higher -3db rolloff point. Right??
Either a simple two resistor voltage divider or a volume pot could be mounted on the amp itself, at the input. This would be great if the preamp was always used with the same amp. But this seems like an unlikely scenario.
A more versatile solution would be to build the voltage divider into an interconnect which would be used exclusively with this preamp. Or any other pre that's either too noisy or has too much gain.
One question. If the voltage divider was put on the amp end of the interconnect would it be better to use much larger resistance values than 68k in series with 22k to ground? Wouldn't the 22k to ground be in parallel with the amp's existing input impedance (let's assume 100k)? Or would the 68k, 22k and 100k all be in parallel? Either way that would lower the input impedance significantly (to ~15k) and make the amp harder to drive.
It that's the case than you could use a combo of 2.2 meg in series and a 750k to ground to get about the same voltage/noise reduction. Or would this create other issues??
Obviously, if the hum is the result of a ground issue then you could eliminate the voltage divider as long as the increased gain is not an issue.
Edits: 02/17/25
All good ideas.
| retro-thermionic |
He could just fix the hum problem that started all of this and then he wouldn't need the attenuation at all. Impedance problem solved.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
I tried for months but to no avail. This allows me to enjoy something I have spent a huge amount of time on.
| retro-thermionic |
I hate when that happens.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
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