I have an Audio Innovations s300 that I really like.
However, the gain is too high using a 2V line input. I just move the pot from the intial 7 to 8 o'clock positon and it's already too loud.
Besides using resistors before the volume knob to attenuate, is there a better way to reduce the gain?
Please see the attached schematics of the amp and preamp section.
It'a pp ecl86 with no feedback. I hope those familiar with tube amps could provide some advice. Thank you.
However, the gain is too high using a 2V line input. I just move the pot from the intial 7 to 8 o'clock positon and it's already too loud.
Besides using resistors before the volume knob to attenuate, is there a better way to reduce the gain?
Please see the attached schematics of the amp and preamp section.
It'a pp ecl86 with no feedback. I hope those familiar with tube amps could provide some advice. Thank you.
Attachments
The power amp schematic, most definitely, shows a GNFB loop. Closed loop gain is controlled by the 15 K/100 R NFB pair. Decreasing the 15 K value increases the % of the O/P voltage fed back, which the reduces closed loop gain. Doing that requires the 540 pF. phase compensation cap. to be reworked with a 2 KHz. square wave and oscilloscope.
The increased error correction signals associated with a closed loop gain change could easily saturate the O/P transformer's core. You deal with that matter by reducing the open loop gain. Reducing the open loop gain requires changing V3 from a high μ 12AX7/ECC83 to a medium μ 12AY7. However the ripples in the pond continue to spread, as parts value changes are needed.
The simple, if imperfect, solution to the problem is to "pad" the CDP's signal down by incorporating 1/8 W. 1% metal film resistors (which form a voltage divider) inside the RCA plugs, at the the integrated amp end. The total resistance in the divider should be either exactly 10 K or slightly greater. Commercial 2 VRMS CDPs must be able to drive the IHF "standard" 10 K load and the divider will be just fine working into 100 K or greater.
BTW, there are nits to pick in that power section's topology. It employs a paraphase style phase splitter, which is out of favor. The 470 K resistance at the I/P combines with the large Miller capacitance of the 'X7 triode to form a low pass filter that can degrade high frequency performance. Current best practice, when using 'X7s, is 100 K, for that position. The problem with doing that is adversely loading the phono section down. Again, there are ripples in the pond.
Does the unit use a printed circuit board (PCB) or does it employ point to point (P2P) wiring of some kind? Photos of the interior could help the membership make better recommendations.
The increased error correction signals associated with a closed loop gain change could easily saturate the O/P transformer's core. You deal with that matter by reducing the open loop gain. Reducing the open loop gain requires changing V3 from a high μ 12AX7/ECC83 to a medium μ 12AY7. However the ripples in the pond continue to spread, as parts value changes are needed.
The simple, if imperfect, solution to the problem is to "pad" the CDP's signal down by incorporating 1/8 W. 1% metal film resistors (which form a voltage divider) inside the RCA plugs, at the the integrated amp end. The total resistance in the divider should be either exactly 10 K or slightly greater. Commercial 2 VRMS CDPs must be able to drive the IHF "standard" 10 K load and the divider will be just fine working into 100 K or greater.
BTW, there are nits to pick in that power section's topology. It employs a paraphase style phase splitter, which is out of favor. The 470 K resistance at the I/P combines with the large Miller capacitance of the 'X7 triode to form a low pass filter that can degrade high frequency performance. Current best practice, when using 'X7s, is 100 K, for that position. The problem with doing that is adversely loading the phono section down. Again, there are ripples in the pond.
Does the unit use a printed circuit board (PCB) or does it employ point to point (P2P) wiring of some kind? Photos of the interior could help the membership make better recommendations.
even simpler, use an external passive preamp with volume control or similar, knock the INCOMING signal down, leave the circuit alone.
Thanks everyone for their suggestions.
I could not find any other schematics of this amp. It uses a printed circuit board.
However, I would like to leave the main circuit unchanged since it sounds very good to me. So I think that I'll go ahead with the resistor attenuation.
Also I can probably use a preamp tube with less gain instead of 12ax7?
I could not find any other schematics of this amp. It uses a printed circuit board.
However, I would like to leave the main circuit unchanged since it sounds very good to me. So I think that I'll go ahead with the resistor attenuation.
Also I can probably use a preamp tube with less gain instead of 12ax7?
Changing the input tube will alter the RIAA response significantly, and increase the noise,
unless the circuit is redesigned. Probably the output tube as well.
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Changing from a 12AX7 (u = 100) to a 5751 (u = 70) would give about 3 dB reduction per stage (two stages = 6 dB gain reduction total).
The 12AX7 and 5751 are similar enough that you should just be able to "Plug and Play".
The design of the 5751 lowers the transconductance and lowers the u, in order to have very similar plate resistance as the 12AX7.
The 5751 might need a very small adjustment to the Rs and Cs in the RIAA frequency compensation network if you are going to be picky on a test bench.
But you probably will not notice the difference even if you do not readjust. . . .
Just try the 5751.
You will not find any other tube that has similar enough specifications to work even close to the 12AX7 in that preamp other than the 5751.
If you need more than a 6 dB reduction in gain, then any other tube will require significant adjustments to the Rs and Cs in the RIAA frequency compensation network. "That is Plug and Pray".
May you have Happy Listening to a 5751 phono preamp!
The 12AX7 and 5751 are similar enough that you should just be able to "Plug and Play".
The design of the 5751 lowers the transconductance and lowers the u, in order to have very similar plate resistance as the 12AX7.
The 5751 might need a very small adjustment to the Rs and Cs in the RIAA frequency compensation network if you are going to be picky on a test bench.
But you probably will not notice the difference even if you do not readjust. . . .
Just try the 5751.
You will not find any other tube that has similar enough specifications to work even close to the 12AX7 in that preamp other than the 5751.
If you need more than a 6 dB reduction in gain, then any other tube will require significant adjustments to the Rs and Cs in the RIAA frequency compensation network. "That is Plug and Pray".
May you have Happy Listening to a 5751 phono preamp!
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How is is the 2 volt test signal achieved? If it is from the pre-amplifier then, the simplest method of reducing the gain is to remove the 100mFd capacitor in the cathode circuit of the V2 stage in the phono pre-amplifier
The power amplifier does use overall negative feedback as Eli Duttman has pointed out and if you need to drop the gain further, the solution may be to convert the paraphase phase splitter into a long tail pair circuit.
The power amplifier does use overall negative feedback as Eli Duttman has pointed out and if you need to drop the gain further, the solution may be to convert the paraphase phase splitter into a long tail pair circuit.
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By reducing the gain, and hence the Miller input capacitance of the tube,
the RIAA response would be altered. The circuit would need redesigning.
to flo777
I havent' understood if you have the issue for phono stage only or/and line stage.
Walter
I havent' understood if you have the issue for phono stage only or/and line stage.
Walter
rayma,
Inner layer of the onion versus the outer layer of the onion:
Well, you made me calculate whether the phono preamp RIAA network needs to be redesigned when changing from a 12AX7 to a 5751.
The 12AX7 output stage triode gain is about 67. With 1.7pF grid to plate, the Miller Effect C is 114pF.
The 5751 output stage gain is about 47. With 1.7pF grid to plate, the Miller Effect C is 78pF.
Compare the 22,000 pF (22nF) to ground across the circuit that is in parallel with either 114pF or 78 pF. Those are 0.5% and 0.4%.
That is a 0.1% change.
I bet those 22,000 pF caps in the circuit are not specified to be nearly as good as +/- 0.5%.
Do use the 5751 in the preamp, and call it good.
Inner layer of the onion versus the outer layer of the onion:
Well, you made me calculate whether the phono preamp RIAA network needs to be redesigned when changing from a 12AX7 to a 5751.
The 12AX7 output stage triode gain is about 67. With 1.7pF grid to plate, the Miller Effect C is 114pF.
The 5751 output stage gain is about 47. With 1.7pF grid to plate, the Miller Effect C is 78pF.
Compare the 22,000 pF (22nF) to ground across the circuit that is in parallel with either 114pF or 78 pF. Those are 0.5% and 0.4%.
That is a 0.1% change.
I bet those 22,000 pF caps in the circuit are not specified to be nearly as good as +/- 0.5%.
Do use the 5751 in the preamp, and call it good.
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waltube,
Yes, lets find out from flo777.
Is gain reduction needed in the preamp?
Is gain reduction needed in the power amp? There may be other signal sources, a tuner, a CD player, etc.
Or is gain reduction needed in both? (maybe a very high output phono cartridge), (and multiple signal sources).
A 5751 in the preamp might cost a little, but so does a quality matched stereo potentiometer in the power amplifier.
A pair of fixed resistors per channel as a voltage divider is less expensive than either.
Yes, lets find out from flo777.
Is gain reduction needed in the preamp?
Is gain reduction needed in the power amp? There may be other signal sources, a tuner, a CD player, etc.
Or is gain reduction needed in both? (maybe a very high output phono cartridge), (and multiple signal sources).
A 5751 in the preamp might cost a little, but so does a quality matched stereo potentiometer in the power amplifier.
A pair of fixed resistors per channel as a voltage divider is less expensive than either.
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Yes, both the phono and line stages have too much gain.
I just mentioned the line stage with 2V input because that's what I use the most.
By the way, there is a single 12at7 tube in the line preamp.
I just mentioned the line stage with 2V input because that's what I use the most.
By the way, there is a single 12at7 tube in the line preamp.
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A schematic of the line preamp would definitely help.
As "rayma" stated, the volume control's value is important. The RCA plug resistor "trick" will not impact on both a line level source and phono. Knowing the volume control's value allows for correct specification of a "padding" resistance that's wired in series with it.
BTW, what is the sensitivity of the speakers being used?