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 Post subject: Re: Baxandall circuit
PostPosted: 31 May 2015, 11:24 
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Tjeu wrote:
One question: Why not employ a cathode-follower configuration at the very end; it would result in a much lower output impedance to the stages that will follow?
A just question. And one worthy of a good answer.

If you look at the thread here on building a 4S with a CF output, you'll see that this is exactly what i did. The prime requirement in this other thread was for a buffered stage with low output impedance. The resultant circuit has Ro≈1.4kΩ. It's a good buffered preamp with low output impedance.

This project is a little bit different. The primary purpose here is tone control, which presents a few design challenges. The operation of a tone stack presents wildly varying impedances at the input and output based on frequency and tone control settings. So in general, it requires isolation from the upstream and downstream stages. It also presents a significant loss in signal (≈21dBv in this case) with tone controls at the nominal settings. As such it generally requires a gain stage to make up the lost signal. This situation leads to a three stage circuit: a gain stage, a tone control stage, and a buffer stage.

In addition, I made an arbitrary decision here about how much gain should be in the gain stage. I decided that for maximum flexibility, I would incorporate a 4S universal stage as the gain stage. This gives the user more options for overall gain without changing the circuit. With this decision made, the next question is to pick the topology. The choices here are (unless I wanted to a add a fourth stage, which I did not) "Buffer→Tone Stack→Gain" OR "Gain→Tone Stack→Buffer". Now a properly designed vacuum tube cathode follower has both a very high input impedance and a low output impedance. As such, it can go on either the input or the output without much problem. So the real question is what happens when I connect a 4S gain stage to the tone stack.

So, as you have indirectly pointed out above, the output impedance of the 4S stage is not so low as a CF and it varies from tube to tube. The 4S Universal output impedance is as low as 11.5kΩ when using a 12AU7 and as high as 38.7kΩ when using a 12AX7. So the first question is what happens when the gain stage is on the input to the tone stack. The answer is it's not pretty. The following two plots show the response of the combined 4S preamp and tone stack when the 4S is on the input for both the bypassed and unbypassed cathode configurations.
Attachment:
Bypassed.png
Attachment:
Unbypassed.png
As you can see, there is significant variation in the upper frequencies when shifting between tubes, There is also significant variation with controls in the nominal settings. I was (and am) unwilling to settle for such performance in one of my designs. As such, I decided that the buffer should go on the input and the gain stage on the output. Compare these two plots with those I posted above. You can see that the ones above are far better behaved.

Having the CF on the input also has some other advantages. Even with the volume at full, it is virtually impossible to drive the circuit in to overdrive. Combined attenuation prior to getting to the gain stage is about -22.4dBv. This means that even with the 12AX7 in the gain stage, it will require a full 15v peak to overdrive the circuit (29v peak with the 12AU7). Also the input impedance of the chain is driven entirely by the volume control (500kΩ).

One major disadvantage is of course, the output impedance is not quite a low as it could be. However, given the design constraints under which the design was done, I still think that this is a good solution. These output impedances are more than suitable for the vast majority of tube equipment out there.

Does this answer your question?


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 Post subject: Re: Baxandall circuit
PostPosted: 31 May 2015, 13:21 
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Thank you Matt, and yes I can follow you, almost ;-)

My knowledge is mostly theoretic and i still am getting acquainted with your well sounding system-names like 4S.
I am reading a lot these days, believe me.
First thing to do now is to read the complete thread, and that will be a pleasure to me.

Also the name "Tone-stack" came completely new to me, in the old days we spoke about Bass- and Treble-control.

I would like to look at my amplifier system to be built in modules like Phono / Micro pre-stage, general pre-amp, tone-stack, powerstage.
I was thinking about the tone-stack to be active in the sense that there will be no loss and the total system with or without stack would stay the same; "Defeat" as you call it.
Your Baxandall looks very active indeed.
But when you use the defeat switch, the gain will be higher so you have to lower the volume?

What would happen if your Bax control in neutral setting (both High and Low at 50%) would give 0dBv at the output in stead of the less then -20dBv now?
In that configuration it would become a neutral building-block with tapers on neutral, especially when you start and end it with CF, you could put it between your pre-amp and end-amp or you could leave it out / defeat it.
I have been fiddling around with Duncan's Tone Stack Simulator; it's a great thing!

Mathieu


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 Post subject: Re: Baxandall circuit
PostPosted: 31 May 2015, 15:42 
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Tjeu wrote:
But when you use the defeat switch, the gain will be higher so you have to lower the volume?
As the main schematic is drawn, yes. However, look at the schematic again and at the bottom is a diagram labeled "Optional Pad in Defeat Line". By adding these two resistors, there is virtually no gain difference (with the controls at neutral positions) between the two switch settings.
Tjeu wrote:
What would happen if your Bax control in neutral setting (both High and Low at 50%) would give 0dBv at the output in stead of the less then -20dBv now?
In that configuration it would become a neutral building-block with tapers on neutral, especially when you start and end it with CF, you could put it between your pre-amp and end-amp or you could leave it out / defeat it.
It is very important to pay attention to the plot axes in the diagrams. The tone stack itself (i.e. just the resistors, potentiometers, and capacitors) cannot ever boost bass or treble. The nominal insertion loss of the tone stack will always be GREATER than the highest bass or treble "boost" value. This is simple physics. The "boost" and "cut" values are always relative. This is why the gain of the tone stack is -21dBv. It is impossible to build a passive tone stack with 0dBv insertion loss at nominal settings. The tone stack itself, with a cathode follower on both the input and output would have approximately 23 dBv of end-to-end loss.

Below is a gain diagram showing the end to end gain of the entire schematic I've posted above.
Attachment:
End-to-End Gain.png
As you can see, the gain stage is required to overcome the losses at other points in the circuit. With a the volume at max and a 12AU7 in the 4S universal stage, the overall end-to-end gain is only 1dBv. This is essentially a neutral gain block WITH THE GAIN STAGE. If you use a higher gain tube then you can get higher gain, but not by a large amount.

As the saying goes... "There is no such thing as a free lunch."


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 Post subject: Re: Baxandall circuit
PostPosted: 31 May 2015, 16:20 
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Thank you Matt!

When I mentioned a active tone-control / tone-stack I was arguing that some gain has to be inserted to keep it neutral.
You made things very clear.

So, when i throw some small schematic to you incidentally, you could SPICE it?

Your (almost) name sharer,

Mathieu


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 Post subject: Re: Baxandall circuit
PostPosted: 31 May 2015, 16:52 
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While digging the Internet on Baxandall i found this:
http://www.angelfire.com/electronic/fun ... 6-A-A-back

Mathieu


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 Post subject: Re: Baxandall circuit
PostPosted: 31 May 2015, 18:49 
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What I like about the defeat switch is it allows and quick and simple A\B between tone settings and flat. On the fly in real time you can switch between a selected tone setting and dead flat.

Or if you like a certain tone setting for some music but not others you are not resetting the controls everytime you change music. This could even lead to two selectable tone stacks but with knobs with a vernier marked on them should allow quick resetting of favourite settings.

If you remember some LPs of old have three small clock like images on the back where you could score your tone and volume settings. This preamp may just bring them back, lol.

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 Post subject: Re: Baxandall circuit
PostPosted: 31 May 2015, 20:40 
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Tjeu wrote:
While digging the Internet on Baxandall i found this:
http://www.angelfire.com/electronic/fun ... 6-A-A-back

Mathieu


THANKS! :up:

I had lost that bookmark on a fubared backup. Nice to have it again :smoking:

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 Post subject: Re: Baxandall circuit
PostPosted: 02 Jun 2015, 08:52 
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I think that I will put in the production line Matt's proposal. But, my question is the following:

Will the power supply prepared for original 4S pre-amp , be suitable for tone control that has two tubes instead of one? I will stay with 250V but is it ok to use my toroidal transformer 190-0-190 100mA?

And something else to avoid a mistake. My secondary is 3.15-0-3.15 ; how should I connect it in order to get 6.3 Amps? Can I use center tap instead of virtual CT described in 4S power supply?

Thanks for any advice


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 Post subject: Re: Baxandall circuit
PostPosted: 02 Jun 2015, 19:13 
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dimitris1811 wrote:
Will the power supply prepared for original 4S pre-amp , be suitable for tone control that has two tubes instead of one? I will stay with 250V but is it ok to use my toroidal transformer 190-0-190 100mA?
The cathode follower only draws about 1.5mA per channel. The power supply should have no problems with that. As for voltage, the circuit was specifically designed for 300v B+. You can use 250v but the performance won't be quite as good. I don't really see any reason to not use the transformer you have on hand.
dimitris1811 wrote:
My secondary is 3.15-0-3.15 ; how should I connect it in order to get 6.3 Amps? Can I use center tap instead of virtual CT described in 4S power supply?
Just connect the filaments to the 3.15v lines. Use the center tap as the signal ground connection. Then you won't need the virtual center tab with the two 100Ω resistors.

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 Post subject: Re: Baxandall circuit
PostPosted: 03 Jun 2015, 00:59 
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Thanks Matt. What is the voltage rating of 4DPT switch? I found up to 125 VAC/DC but should it be 300?


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