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 Post subject: Re: Baxandall circuitPosted: 30 May 2015, 11:05
 Project Author

Joined: 06 Apr 2009, 10:08
Posts: 1620
Location: US Pacific Northwest
Geek wrote:
I've just used a voltage divider around 100K in impedance
Unfortunately, here it's not quite so simple. The kicker is the volume control in this position. The upper frequency rolloff depends on the volume position. At ≈80% setting the high end will be attenuated between 2.2dBv and 16dBv dependent on the tube. In the tone stack in position, it's even more variable.

The problem is that there are four degrees of freedom in the operation of this circuit. And at least three of them are interdependent. As such, I have some more design work to do to make it better behaved at across the entire operational space (high to low bass, high to low treble, high to low volume, & different output tubes). I'm sure I can do it. It will just take some more thinking about the layout and some more detailed calculations including more factors in the equations.

Stand by for more...

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Matt

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 Post subject: Re: Baxandall circuitPosted: 30 May 2015, 19:50
 Project Author

Joined: 06 Apr 2009, 10:08
Posts: 1620
Location: US Pacific Northwest
GOT IT!

It was the position of the volume control which was making things too variable. By moving the volume control to the front of the chain everything works out properly and the pad for the defeat line becomes simple. Now using a single 4PDT switch, the tone stack can be shifted in and out without a gain shift in the preamp. First, here is the circuit.
Attachment:
Schematic BEST (CF Input - 4S output) small.jpg

Now here are the embedded response plots with lines for 12AU7, 12AT7, and 12AX7. The 12AV7 and 12AZ7 fall within these three. These are plots with the bass and treble controls at extreme cut and boost compared to the nominal setting. The only deviation is about a 1dB shift in the peak treble boost between the 12AU7 and the 12AX7. Frankly no one would even know this was there from just listening to the unit.
Attachment:
Response Curves 1.png

Attachment:
Response Curves 2.png

And with the tone stack defeat switch in the "Defeat" position, the interstage response looks like this.
Attachment:

So now the whole thing should work just as intended without weird tone shifts when switching between tubes or flipping the "Tone Stack Defeat" switch.

As always, questions and comments are more than welcome.

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Matt

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 Post subject: Re: Baxandall circuitPosted: 30 May 2015, 20:15
 Project Author

Joined: 06 Jun 2008, 18:23
Posts: 5189
Location: Australia
Matt, absolutely great. This is a top little preamp stage now. Bringing back tone controls, I never would have imagined it. I must take a copy of the final cct. for a future build. Currently building a tube phono stage.

I could be cheeky and say what about a balance control? Thinking back to early integrated SS amps you had bass, treble and balance as additional controls. My friend's old gold coloured Marantz had a mid-band control also. All on sliders.

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 Post subject: Re: Baxandall circuitPosted: 30 May 2015, 21:10
 Project Author

Joined: 06 Apr 2009, 10:08
Posts: 1620
Location: US Pacific Northwest
mwhouston wrote:
I could be cheeky and say what about a balance control?
I always considered balance controls cheating. I much prefer independent volume controls for each channel; much more flexible.

My build of this preamp will have six controls. Independent volume, bass, and treble controls for each channel.

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Matt

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 Post subject: Re: Baxandall circuitPosted: 31 May 2015, 03:14

Joined: 08 Aug 2009, 03:11
Posts: 2229
Location: Chilliwack, BC
Suncalc wrote:
I always considered balance controls cheating. I much prefer independent volume controls for each channel; much more flexible.

I've switched to that too.

It has shown to be the preference of our customers as well. Two volume controls makes them feel just fine with an imbalanced tube pair. A volume + balance makes them "OCD" when the balance is anything but centre. Positive psychological marketing for dual volume controls

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* Ratings are for transistors - tubes have guidelines*
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 Post subject: Re: Baxandall circuitPosted: 31 May 2015, 10:23

Joined: 25 Jan 2015, 11:51
Posts: 54
Location: Netherlands
Trying to follow this; very interesting indeed!
It gives me inspiration for my next project.

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?

Mathieu

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 Post subject: Re: Baxandall circuitPosted: 31 May 2015, 11:24
 Project Author

Joined: 06 Apr 2009, 10:08
Posts: 1620
Location: US Pacific Northwest
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.

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 Post subject: Re: Baxandall circuitPosted: 31 May 2015, 13:21

Joined: 25 Jan 2015, 11:51
Posts: 54
Location: Netherlands
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 circuitPosted: 31 May 2015, 15:42
 Project Author

Joined: 06 Apr 2009, 10:08
Posts: 1620
Location: US Pacific Northwest
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 circuitPosted: 31 May 2015, 16:20

Joined: 25 Jan 2015, 11:51
Posts: 54
Location: Netherlands
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.

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

Mathieu

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