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In Reply to: RE: Poor Design for a Preamp posted by FlaCharlie on February 16, 2025 at 12:17:02
You can't drive into a 43 ohm load to ground with a stage that has a 8.3k output impedance. That would rotate the load line for the tube way to the vertical and there would be very little gain and a whole bunch of harmonic distortion.
8.3k output wants to see a 83k load.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Tre made the suggestion to use 68k+22k resistors to give me a dropping output level but not to limit upper frequency. Tried this ides with a 6N7 preamp I also had noise issues with and it worked well.Even with my DIY shieldless lead (175pF) into 100K upper 3db was 37KhZ. 6N7 are power tubes and Im not sure of they have a very high frequency range or not. Regardless I'm happy with 37K.
Applying the same 68k+20k idea to the 01a preamp gave me noise level outputs of 1.9mV and 0.9mV a big improvement over the ~4.0mV with the 300k+100k setup. At good volume only by putting my ear to my 93db sensitive 12" woofers do I hear the slightest hum then only in the left channel.
Good result for two preamps so far and two to go. Lucky I have a good collection of Rs to mod the others.
| retro-thermionic |
"Tried this ides with a 6N7 preamp I also had noise issues with and it worked well.Even with my DIY shieldless lead (175pF) into 100K upper 3db was 37KhZ. 6N7 are power tubes and Im not sure of they have a very high frequency range or not. Regardless I'm happy with 37K."
The 6N7 was primarily intended as a Class B output tube. But the 6N7 data sheets (GE, TungSol) suggests multiple uses for the tube including use as a Class A driver and gives suggested operating points for that application. Since nobody in the DIY community seems interested in building Class B amps all the forum discussion on the 6N7 seems to be about using it as an input / preamp tube.
Apparently, there are some glass (G, GT) versions. I have some of the 6A6s which are glass. The 6E6, 79 and 53 are supposedly equivalents or near equivalents. There's also the 5694 which is equivalent or nearly but has separate cathodes.
At one point I was considering building a Japanese SE 2A3 design that uses them on the input but I was going to use the 6A6 version. I like using less common tubes but I don't really have a need for a preamp with that much gain (mu 35).
I have priced on eBay some glass 6N7s and though not cheap well within my budget. They do look good though. But still having three boxes of tubes im trying not to buy more but use up what I have in new DIY gear and move on. I have a number of other metal tubes. All sound excellent in their respective amps.
| retro-thermionic |
Yeah, I don't remember if the 6A6s I got came from eBay or if I found them locally.
I'm less than 30 minutes from Radio Electric Supply (vacuumtubes.net) which is probably the biggest tube dealer in the world. They have warehouses full of tubes, many millions of them, though not all audio related, of course. They ship worldwide and have reasonable prices.
They're generally much cheaper than the "boutique" sellers who give flowery descriptions of how a particular tube allegedly sounds. As if a particular tube sounds the same regardless of which circuit it's used in or any other differences in the rest of the system. Of course, the "holy grail" tube$ are always what they just happen to have in $tock at any particular time.
The guys at RES (brothers, Roy and Dale) are not audiophiles, they just sell tubes. Hence the total lack of BS.
Another very large dealer is about 100 miles south of me in Orlando. That's vacuumtubesinc.com which also ships worldwide.
The only metal tube I've experimented with is the 6AG7. It caught my interest because it's incredibly easy to drive. I have a breadboarded PSE design that uses them with each channel driven by half of a 3A5, which is a dual DHT with a mu of 15.
It's another "inverted SET" (iSET) design which is a term coined by Andy Evans. I built another iSET amp which I call the Nuance. Details are in a thread on Audio Karma.
The "inverted" aspect refers to fact that instead of using DH output tubes driven either by higher mu indirectly heated tubes or multiple stages of somewhat lower mu tubes, the amp is only two stages with input tubes that are DH. The outputs are indirectly heated and to easy to drive, unlike the output tubes that are commonly used in SET amps.
The PSE 6AG7 is on the sidelines for now, though. I'm back to playing with low mu (10 or less) preamp tubes at the moment which is why your thread caught my eye.
"Tre made the suggestion to use 68k+22k resistors to give me a dropping output level but not to limit upper frequency."
Actually, I was the one who suggested using the combination of 68k in series and 22k to ground and said it should improve the -3db point of the upper frequencies.
My earlier post: "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?"
Tre quoted me and commented about the corresponding -1db point, which he said would be 27.375kHz. He suggested that, from a technical standpoint, this would still not be enough to "get the filter completely out of the audio band".
I'm glad to hear that, from a practical standpoint, it seems like it improved things subjectively. I try to understand the technical end of things - hence my questions in this thread - but I'm not convinced that striving for technical perfection is always audible beyond a certain point. Sometimes "better than it was" is good enough and going to more extreme lengths is just a waste of time and money if it doesn't yield audible improvement. IMO, but that's just me.
I guess for an engineer technical perfection IS the goal so it's a valid pursuit regardless of whether it's audible or not.
I understand how it's easy to get confused though. AA's software is ridiculously out of date and totally lame. Unlike other sites like DIY Audio, Audio Karma, and others AA doesn't allow you to easily quote a previous post without rewriting it and adding quotation marks. As I have done above.
Unless someone makes the effort and adds a quote manually it's often impossible to tell which previous post someone is responding to.
I suspect that AA will never modernize their software. I don't know about the site as a whole but the forums I used to visit regularly years ago are pretty much dead in the water now. Some of them now go weeks without a post where, in the past, there were a large number of new posts and threads every day. Sad.
I still find some useful and interesting posts on AA but I rarely post here anymore because of the lack of activity. Fortunately, the search function yields a wealth of information from the time when AA was a vibrant forum.
Well thanks for the suggestion. Probably like you Im on a number of forums mainly an Australian one which is where I live. Id like to think we are all here to listen and to help where we can fostering friendship and help in a common field of interest.
Two preamps to re-mod. Ill post results.
| retro-thermionic |
"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?
"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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