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Old 31st December 2010, 04:19 PM   #1
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Default Mini-console Schematic

I have mentioned in passing a mini-console project that I am working on. As I am getting ready to start drilling the chassis for the PA section I thought I would post the intended schematic including the preamp section. I am sure that some details will change as I test the results.

The preamp (12AX7 and 12AU7s) will be on a separate small chassis salvaged from a hammond L-101. Power supply will be 5AR4 rectifier with CLCLC filtering. The speaker drivers will be MA CHR-70 (series one) in BR tuned to get solid output down to a bit below 40Hz.

One concern is that some subwoofer amps may not be able to adjust crossover down low enough to avoid a boomy bump when trying to integrate with the mains. I may try to add a switchable HP filter to help with that.

Comments welcome.

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Old 14th February 2011, 11:08 PM   #2
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I just realized last night that this won't really cut it for my intended use. The primary input source is an iPod which in my experience has less output voltage than a CDP. I figure maybe 1V peak (1.4V RMS) which is no where near enough to drive this to full output.

It has been suggested to me that using pentode drivers would work better for the shade FB and I should think would also have more gain but I question whether even that would be enough. The preamp section above actually has a gain of slightly less than unity. I suspect that the easiest solution is to drill out an unused 7 pin socket on the preamp chassis and install an additional 9 pin socket which I could use for an additional gain stage (say 6N1P) immediately after the tone control section.
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Old 15th February 2011, 04:58 PM   #3
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Since your tone control is a negative feedback type you don't really need to drive it from a very low source impedance like a cathode follower. So an alternative would be to change the first stage into a common cathode gain stage.

Cheers

Ian

Last edited by ruffrecords; 15th February 2011 at 04:59 PM. Reason: spelling correction
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Old 15th February 2011, 05:28 PM   #4
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Quote:
Originally Posted by ruffrecords View Post
Since your tone control is a negative feedback type you don't really need to drive it from a very low source impedance like a cathode follower. So an alternative would be to change the first stage into a common cathode gain stage.
Opposite, actually... Parallel feedback makes it look as if it is loaded on a virtual ground.
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Old 15th February 2011, 11:01 PM   #5
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Quote:
Originally Posted by Wavebourn View Post
Opposite, actually... Parallel feedback makes it look as if it is loaded on a virtual ground.
With the circuit as shown that never happens. First each input leg has a 56K in series so the worst case load can never be worse than these two in parallel. Second, the feedback network is modified from the straight forward shunt derived shunt applied feedback by the addition of large series resistors from the sliders of the pots to the grid of the 12AX7. On that basis I would estimate the worst case load to be no less than 100K which a CC 12AU7 stage should have no problem driving.

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ian
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Old 16th February 2011, 12:10 AM   #6
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Quote:
Originally Posted by ruffrecords View Post
With the circuit as shown that never happens.
Your guess is wrong again. Parallel feedback decreases input resistance. Ref: Ohm's law.

Or, here is the picture I drew for students many years ago, to explain them feedback basics: hope it helps...

www.wavebourn.com • View topic - Feedback basics

Edit: similarly, U8 (6N1P) is loaded not only on 47K in parallel with 220K, but also on 270K/8 (I assume gain of KT-88 stage is 8). Plus, since gain of KT88 is non-linear, it is loaded on non-linear resistance.
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Last edited by Wavebourn; 16th February 2011 at 12:21 AM.
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Old 16th February 2011, 09:47 AM   #7
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I agree about U8 but I am referring to the NFB around U2 the 12AX7. Referring to your NOOBs guide to NFB that you so thoughtfully provided a link to, what we need to identify is the value of R1 in your little op amp circuit below, as that is effectivekly what defines the input impedance in this instance.

http://www.wavebourn.com/images/op_amp_neg.gif

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Old 16th February 2011, 10:35 AM   #8
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Although the input impedance of the tone control is reasonably high, as ruffrecords says, it does vary with both frequency and tone control position. At centre, LF Z is 181K (56K+125K), HF it is 43K (56K||181K). This is why it needs a low impedance drive. However, I would have thought that the anode impedance of a common-cathode 12AU7 is low enough to suffice - after all, this is not hi-fi!
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Old 16th February 2011, 04:23 PM   #9
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Quote:
Originally Posted by DF96 View Post
Although the input impedance of the tone control is reasonably high, as ruffrecords says,
Actually, my correction was about the statement that input impedance is high because it is a tone stack in feedback loop. It was incorrect.

Quote:
Originally Posted by ruffrecords View Post
Since your tone control is a negative feedback type you don't really need to drive it from a very low source impedance like a cathode follower.
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Old 16th February 2011, 05:24 PM   #10
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Yes, I agree that the tone circuit is loaded by a virtual ground. The issue is input impedance. This is high, but unfortunately too variable.

Note that the high value resistors from sliders to grid do not raise the input impedance, as the grid is not the virtual ground. The sliders jointly form the virtual ground. The high value resistors merely reduce the open-loop stage gain, and isolate the two pots from each other.
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