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Old 31st March 2013, 11:07 PM   #41
Cassiel is offline Cassiel  Libya
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I did make some effort to content the engineering side of this thread -me posting graphs? you got to be kidding me!- but to no avail. Boomy responses was all I got. I'm satisfied with the end result though, did learn a few things and, in general, this thread has been fun to read. My preamp doesn't have a better bass because of it but this doesn't trouble me. Maybe I should repeat again: I don't have a problem with my bass, just not the best one around and that is all.
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Old 1st April 2013, 12:30 AM   #42
gootee is offline gootee  United States
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Well, someone quoted a 6.3 Hz cutoff frequency from a 47 uF cap. And another mention was made of a 5 Hz cutoff frequency. Those -3 dB frequencies do seem rather high, if you're worried at all about bass response.

Usually, you would want a -3 dB cutoff frequency that is no more than 1/10th (or 1/100th for purists) of the lowest frequency that you want to remain unaffected.

So maybe you should triple those capacitances that were mentioned, to get the cutoff frequencies down to 2 Hz or less, so your bass might be decent down to 20 Hz. And maybe it would be quick and easy to test by just temporarily clamping a 100 uF cap in parallel with the 47 uF, and so on (but do them all at once, not one at a time).

I didn't look at the circuit (and am not a tube circuit expert, anyway) so hopefully someone will chime in if any of those changes would also require something else to be changed.
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Old 1st April 2013, 05:24 AM   #43
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Quote:
Originally Posted by Loren42 View Post
I am not sure where you get your information, but that is untrue for a line stage amp. The difference in frequency response between vacuum tubes and solid state will be indistinguishable.

What exactly is untrue? That there is an audible difference?

Frequency response says very little about anything, especially for a line stage where obtaining a level response is trivial. Btw, most solid state preamps are dc coupled, so on top of differences in active devices, tube amps usually have coupling caps with their specific sonic contributions.
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Old 1st April 2013, 03:44 PM   #44
DF96 is offline DF96  England
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Quote:
Originally Posted by analog_sa
Frequency response says very little about anything
You couldn't make it up! I guess gain, noise, hum and distortion tell us very little too.
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Old 1st April 2013, 04:46 PM   #45
Loren42 is offline Loren42  United States
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Quote:
Originally Posted by analog_sa View Post
What exactly is untrue? That there is an audible difference?

Frequency response says very little about anything, especially for a line stage where obtaining a level response is trivial. Btw, most solid state preamps are dc coupled, so on top of differences in active devices, tube amps usually have coupling caps with their specific sonic contributions.
Well, first, two competently designed line stages, regardless of topology (solid state or valve), should be indistinguishable over the audio spectrum.

Yes, DC coupled circuits can pass 1 Hz signals, but 1 Hz or even 10 Hz does not impact audible bass for music. The lowest a piano goes is 27.5 Hz and you won't hear anything below about 20 Hz anyway.

Second, there is no magic in solid state that artificially boosts bass any more than there is a deficiency in vacuum tubes that suck bass out. Again, a poorly designed line stage can do that and it can be done regardless of topology.

Third, any distortion injected by components such as capacitors is just that; distortion. It is not going to suck out bass - unless it is incorrectly sized for the job. However, I already stated that we are comparing competently designed circuits, not something designed correctly to something designed incorrectly. That comparison, for the sake of arguing topology, is useless.

However, if you have some scholarly evidence that supports your claim that vacuum tubes can not pass audible bass frequencies as well as solid state, I would be happy to review it.
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Old 2nd April 2013, 04:41 AM   #46
gootee is offline gootee  United States
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Transistors, Tubes Sound The Same, It's The Circuit Topologies That Differ | Analog content from Electronic Design
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Old 2nd April 2013, 11:48 PM   #47
gootee is offline gootee  United States
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Be sure to read the readers' comments, way down below the article.
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Old 5th April 2013, 01:43 AM   #48
gootee is offline gootee  United States
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Quote:
Originally Posted by gootee View Post
Well, someone quoted a 6.3 Hz cutoff frequency from a 47 uF cap. And another mention was made of a 5 Hz cutoff frequency. Those -3 dB frequencies do seem rather high, if you're worried at all about bass response.

Usually, you would want a -3 dB cutoff frequency that is no more than 1/10th (or 1/100th for purists) of the lowest frequency that you want to remain unaffected.

So maybe you should triple those capacitances that were mentioned, to get the cutoff frequencies down to 2 Hz or less, so your bass might be decent down to 20 Hz. And maybe it would be quick and easy to test by just temporarily clamping a 100 uF cap in parallel with the 47 uF, and so on (but do them all at once, not one at a time).

I didn't look at the circuit (and am not a tube circuit expert, anyway) so hopefully someone will chime in if any of those changes would also require something else to be changed.
Cassiel,

This would be so easy to test. Did you lose interest?

Tom
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Old 5th April 2013, 02:32 AM   #49
benb is offline benb  United States
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This is a tube circuit configuration I've not seen before, but tracing out the circuit and especially knowing the values helps to show what's happening.
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Originally Posted by Cassiel View Post
Anyway, even though I haven't had the time to try out possible fixes I have spotted a problem with one tube: one of the triode's filaments was underpowered, seriously underpowered. Barely glowing. The triode with problems was facing the wall so I really don't know when the problem started. Turns out my bakelite socket is acting up; my fault really, I have a soft spot for vintage parts and sometimes **** happens. Now I have to replace the sockets and possibly change the schematic. No more news from me in a while. Thanks to all.
If you must use "old" parts, go for high quality NOS (New Old Stock, unused parts). They might be more expensive, but I know I've got some ceramic miniature tube sockets somewhere that cost me something like a couple dollars each.
Quote:
Originally Posted by Cassiel View Post
Value of R2 is 130K.
Great, now that explains how the circuit works. The tube is acting as a fairly high gain (maybe 20 to 100 - regardless, it's a lot higher than the whole circuit), with a negative feedback path through R2.

This is working like an op amp with the positive input grounded. The signal is going through R1 (100k) to the negative input, and the output is connected back to the negative input through R2 (130k), thus the gain is -1.3.

This circuit also makes the grid a virtual ground, so the input impedance seen by the instrument is 100k (or for an "exact" number, it's in parallel with 1M, so the impedance seen is 90.9k), significantly lower than is good for most electromagnetic pickup instruments. The instrument needs to see 1M, and the 1M to ground just provides an illusion. Due to the feedback resistor, the grid is effectively a very LOW impedance, not a very high impedance that it is in more conventional circuits. If this is meant to be some sort of tube buffer, it's doing a bad job. The effect of this load on the pickup must be taken into account to get an accurate frequency response.

Also, since (once again) the grid is a virtual ground, the impedance the output "Cap" drives is RL in parallel with R2 (56.5k), so the resulting low frequency cutoff should be recalculated (though I don't think that's a problem).

Even more, a bass guitar only goes down to 41Hz, unless it's a 5-string bass with a low B, which is 31Hz.


Musically, it may be this lower impedance loading the pickup that makes the mids magic you like, and it may not be easy to get the lows back the way a fully buffered circuit would without changing the mids. Maybe the lows could be boosted in a second stage added to the output of this one.
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Old 5th April 2013, 01:29 PM   #50
Cassiel is offline Cassiel  Libya
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Quote:
Originally Posted by gootee View Post
Cassiel,

This would be so easy to test. Did you lose interest?

Tom
No, I did change the circuit to SRPP and now it sounds x20 better than the SS preamp so who cares if the bass is a little softer. Anyway, the change has improve dynamics and there's a little more punch so I'm happy now. I have a small problem though, my speakers are VERY sensitive and now I have some high frequency noise. And when the volume pot goes to five there's some whistling... looks like the circuit is prone to oscillation; I'm going to install grid stoppers.

benb - thanks for the analysis. The magic of so much feedback is the low noise - there isn't any! And it sounded better than just having the 6SN7 in parallel with no feedback loop.
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