DIY CAT SL1 preamp - Page 8

HVfanatic · 15th February 2013, 07:59 PM

Discussion is getting more and more interesting.

So, we have to rebias the u-follower, put 1M in, add an input coupling cap...

Do we need to rebias the input valve as well?

I think, I slowly get the point: This is nothing for serious high-end hifi. We can do much better... ;-(

DF96 · 15th February 2013, 08:05 PM

Where is the serious high-end hifi in this thread? All we have is a badly designed circuit with obvious mistakes. Sadly, this seems to be all too common in modern audio.

Dimitris AR · 15th February 2013, 08:36 PM

Ok , it's not high- end , but why it is so badly designed ? , it's good that the feedback is taken without any capacitor and directly to to the first stage , it will be better to place the feedback onto lower value cathode resistor ( of course then we have to make some other changes in the input circuit ) or via an resistor devider , and the input coupling cap it will not solve anything , plus that it will add some phase shift in the circuit .

DF96 · 15th February 2013, 08:52 PM

Quote:

but why it is so badly designed ?

Only the designer (or his employer?) can tell us that.

Dimitris AR · 15th February 2013, 09:03 PM

Sorry for that , by saying """ but why it is so badly designed """ I think in greek and try speak in english , English is not my best , I should have said "why you consider it so bad design ".

HVfanatic · 15th February 2013, 09:31 PM

I think, very low frequency disturbance will not occur. So only bad biasing may be the error. I suggest to reduce the cathode resistors of the second stage. What is an optimum current? May someone propose optimum biasing for this schematic and tubes, would be a very good tutorial... May someone propose the best current, I try to calculate the correct resistor value and explain my ideas...

HJWeedon · 16th February 2013, 12:20 AM

Hi there.

Digging back in my memory-banks, (75 years of them) I remember that good rule of "thumb" is to bias a triode amplifier at about 2/3 of the supply-rail. in other words at 1/3 the voltage across the plate resistor and 2/3 the voltage across the tube.

That is reasonable for a resistive plate-load, but with a second tube as a load, my guess, without having used that configuration, would be mid-way between the supply rail and ground. My guess would be a reduced voltage swing equal to 1/2 the supply voltage, versus 2/3 the supply rail for a resistive load. The advantage of a properly biased tube as a load resistor would be significantly reduced even harmonic distortion.

Unfortunately human hearing is not very bothered by even harmonic distortion, so reducing that type of distortion does not improve the acoustical perception much.

I believe that even harmonic distortion falls into the non-descriptive term "warmth"

Hans J Weedon.

HVfanatic · 16th February 2013, 12:26 PM

Hi Hans,

but your rule just talks about voltage distribution, so it totally depends on choosen anode resistor values.

So my question is: What could be the "perfect" current flowing. Reducing the cathode resistors ramps up the current. How do I determine the optimized current???

In the ImPass preamp, an optimun current was choosen for the 6SN7...

What is desired for the 12AX7?

After I got a good value there, I recalculate the sensing resistor for getting the same drive for the upper tube...

DF96 · 16th February 2013, 02:20 PM

Quote:

Originally Posted by Dimitris AR

Sorry for that , by saying """ but why it is so badly designed """ I think in greek and try speak in english , English is not my best , I should have said "why you consider it so bad design ".

OK. If a design looks like one thing but isn't (in this case looks like mu-follower but isn't) it is often a sign that the designer didn't know what he was doing.

Quote:

Originally Posted by HVfanatic

What is desired for the 12AX7?

Read the data sheet or Vade-Mecum or RCA handbook. These give recommended working points for ECC83/12AX7 as a normal voltage amplifier. 260V is a bit low for a supply rail, but we have an active load so we can aim for about 1mA anode current. That means Rk should be about 1-1.5k. 3k is too small. The 47k can be smaller so it doesn't drop too much voltage. 10k should be fine. Then the 1k to the upper grid should be 1M or thereabouts. Finally, if you want it to be an actual mu-follower take the signal from the upper cathode (although this won't make much difference as it is driving a CF).

Why did the designer pick Rk=3k? I suspect he looked at the datasheet, where 200V and 250V supply options are given. Both use a large anode resistor (to get linearity) so both have to use low anode current and hence a largish cathode resistor. As we have an active load we can use a modified version of the datasheet options for higher supply rail - they run at higher current and get more gain and less distortion. Maybe the designer did not realise this, or just had a bad day. Anyway, having picked a bad value for Rk he then perhaps played around and found that he could reduce distortion by artificially forcing more balance between upper and lower parts of the 'mu-follower' - which by now was no longer a mu-follower but a bad active load. 'Bad' because the 47k drops DC voltage (which we don't want) but does not add much AC anode load (which we do want).

There are two ways to use an SRPP/mu-follower. A genuine SRPP is balanced and designed to match its load. Under these conditions the even-order distortion in the upper and lower valves are cancelled, so it really does act a bit like push-pull. This is the best way to use a non-linear valve such as ECC82/12AU7. For a linear valve such as ECC83/12AX7 or ECC88/6DJ8 it is better to use a mu-follower to provide a high anode load. No need for balance, as the valve is linear enough on its own.

HVfanatic · 16th February 2013, 09:53 PM

Hi,

I got the point with the lower cathode resistor. Could you link the datasheet you used?

What I don't understand properly: I thougt, the drive resistor is determined by the wanted drive to the upper valve. Im my opinion, the 10K brings only 25% of drive, so modulation of the upper valve is poor. Or does the higher current improve the amplification factor of the valve, so that only 10 is needed??? Or does the voltage drop becomes unacceptable now with 47K?

I always thought, the SRPP hast to low drive to the upper valve, because the cathode resistor is only determined by bias requirements, and upper valve modulation is a happy sideffect...

I always thought, u-follower is more perfect push-pull then SRPP... And betafollower would be constant current anode load alike.... MHMMMM...

15th February 2013, 07:59 PM	#71
HVfanatic diyAudio Member Join Date: Feb 2008	OMG... Discussion is getting more and more interesting. So, we have to rebias the u-follower, put 1M in, add an input coupling cap... Do we need to rebias the input valve as well? I think, I slowly get the point: This is nothing for serious high-end hifi. We can do much better... ;-(

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15th February 2013, 08:05 PM	#72
DF96 diyAudio Member Join Date: May 2007	Where is the serious high-end hifi in this thread? All we have is a badly designed circuit with obvious mistakes. Sadly, this seems to be all too common in modern audio.

15th February 2013, 08:36 PM	#73
Dimitris AR diyAudio Member Join Date: Nov 2011 Location: Athens	Ok , it's not high- end , but why it is so badly designed ? , it's good that the feedback is taken without any capacitor and directly to to the first stage , it will be better to place the feedback onto lower value cathode resistor ( of course then we have to make some other changes in the input circuit ) or via an resistor devider , and the input coupling cap it will not solve anything , plus that it will add some phase shift in the circuit .

15th February 2013, 09:03 PM	#75
Dimitris AR diyAudio Member Join Date: Nov 2011 Location: Athens	Sorry for that , by saying """ but why it is so badly designed """ I think in greek and try speak in english , English is not my best , I should have said "why you consider it so bad design ".

15th February 2013, 09:31 PM	#76
HVfanatic diyAudio Member Join Date: Feb 2008	I think, very low frequency disturbance will not occur. So only bad biasing may be the error. I suggest to reduce the cathode resistors of the second stage. What is an optimum current? May someone propose optimum biasing for this schematic and tubes, would be a very good tutorial... May someone propose the best current, I try to calculate the correct resistor value and explain my ideas...

16th February 2013, 12:20 AM	#77
HJWeedon diyAudio Member Join Date: Sep 2005	Hi there. Digging back in my memory-banks, (75 years of them) I remember that good rule of "thumb" is to bias a triode amplifier at about 2/3 of the supply-rail. in other words at 1/3 the voltage across the plate resistor and 2/3 the voltage across the tube. That is reasonable for a resistive plate-load, but with a second tube as a load, my guess, without having used that configuration, would be mid-way between the supply rail and ground. My guess would be a reduced voltage swing equal to 1/2 the supply voltage, versus 2/3 the supply rail for a resistive load. The advantage of a properly biased tube as a load resistor would be significantly reduced even harmonic distortion. Unfortunately human hearing is not very bothered by even harmonic distortion, so reducing that type of distortion does not improve the acoustical perception much. I believe that even harmonic distortion falls into the non-descriptive term "warmth" Hans J Weedon.

16th February 2013, 12:26 PM	#78
HVfanatic diyAudio Member Join Date: Feb 2008	Hi Hans, but your rule just talks about voltage distribution, so it totally depends on choosen anode resistor values. So my question is: What could be the "perfect" current flowing. Reducing the cathode resistors ramps up the current. How do I determine the optimized current??? In the ImPass preamp, an optimun current was choosen for the 6SN7... What is desired for the 12AX7? After I got a good value there, I recalculate the sensing resistor for getting the same drive for the upper tube...

16th February 2013, 09:53 PM	#80
HVfanatic diyAudio Member Join Date: Feb 2008	Hi, I got the point with the lower cathode resistor. Could you link the datasheet you used? What I don't understand properly: I thougt, the drive resistor is determined by the wanted drive to the upper valve. Im my opinion, the 10K brings only 25% of drive, so modulation of the upper valve is poor. Or does the higher current improve the amplification factor of the valve, so that only 10 is needed??? Or does the voltage drop becomes unacceptable now with 47K? I always thought, the SRPP hast to low drive to the upper valve, because the cathode resistor is only determined by bias requirements, and upper valve modulation is a happy sideffect... I always thought, u-follower is more perfect push-pull then SRPP... And betafollower would be constant current anode load alike.... MHMMMM...