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Old 5th July 2012, 10:26 PM   #61
piano3 is offline piano3  United Kingdom
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but to actually put some numbers on it; the anode is loaded by a step down tranformer which has a load of perhaps several hundred kilohms so the impedance seen at the anode is in the megohm range and the mu of the valve is less than 20. About the current mirror I cannot comment as I have not used this circuit and I would normally use very high impedance bjt cascode sinks in such a circuit.
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Old 5th July 2012, 10:34 PM   #62
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I'm too lazy for actually putting some numbers there (except mentioning fact that putting several hundred of kiloohms as load is right path for filling wallets of cable guys)

it's easy to calculate what's input impedance of cathode node , so , according to result , calculate minimum value for bridging caps ......

probably Shoog will chime in ....... but I can't see any necessity for that , because value of that cap I'm seeing as of secondary importance
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Old 5th July 2012, 10:39 PM   #63
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ok, my interest is that I am trying various ways of getting round large value coupling caps when interfacing valve mic preamps with SS equiptment. Shoog's idea is similar to one of the things I am trying.
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Old 6th July 2012, 02:19 AM   #64
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I think that Pianos analysis is correct in this instance - 10uf is probably more than enough when the high anode load is the dominant factor. I added the 1000uf caps to see if they made a difference - which with the stiffer current mirror I was reluctant to say that they did. This was why in the initial discussion when Lars suggested I try at least 470uf to reduce the resonant hump induced by the cathode resistors - I pointed out that this would not be necessary when the current mirrors were in place.

The caveat is that listening tests showed this over analytical presentation and the only way to get rid of it was to either bypass to ground (which I didn't want to do) or try a low impedance load (100R) at the cap junction - for which a junction was necessary. However by our previous analysis it is probably true to say that the caps could be reduced down to 20uf back to back, allowing the use of film caps and creating the needed node.

Listening tests dictate the ultimate design decisions, and the various stages of building this preamp have produced radical shifts in the overall presentation with each change. I think this version is coming to a completed implementation which achieves the goal of hyper detail, absolutely dark quiet background and smooth presentation. I have to add that the high output impedance doesn't seem to have the expected effect on drive capability - it is delivering 300mA to the output load - this gives it massive authority in delivering tight detailed bass. The step down ration is also sufficient to overcome the inherently high capacitances within the transformer.

I was listening to a Kind of Blue a day or so ago and was staggered by the fact that I could almost see the drums been brushed so lightly that they easily get lost in the background with my other preamps.

Shoog

Last edited by Shoog; 6th July 2012 at 02:25 AM.
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Old 6th July 2012, 12:38 PM   #65
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Shoog,

It is great that with the help of semiconductors these inexpensive magnetic parts can perform so well! (and outperform easily available push-pull x-formers) I have a couple of 125 watt ballast chokes(DCR 12 ohms) and I am thinking that they might do for passive LC filtered heater supplies for series heater chains.
I am trying to understand the details of the current mirror; given that your HT supply is unregulated and allowing for the usual spread of valve parameters how did you choose 164R as the programming resistor? Did you just experiment? And what about the 100R collector load resistor?
I am very keen to understand this as I am tired of building negative supplies when I might not need to and which are just more to cram in the cases.
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Old 6th July 2012, 01:06 PM   #66
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A Current mirror as implemented has a few special characturistics - but it is not a CCS as usually implemented. The current mirror enforces current balance on its two loads - but it doesn't set that current within the external load. The DC current is set by the bias which is applied to the valve just as a normal valve is biased. In this case the bias point is defined by the voltage drop across the transistor (0.7V) and then the voltage drop across the resistor. The resistors are chosen such that each transistor drops the same voltage and hence has the same dissipation (for temperature stability). In this case the total cathode bias on each side is -4V which sets the 5687 current at about 15mA. In doing this the impedance of the current mirror is set high which has the effect of blocking the AC signal from been passed by the valve. Also the two loads can only pass the same current (which is effectively the same as having high impedance). So the current mirrors have to be bypassed either to earth in order to allow the signal to pass, or to the other cathode of the LTP to allow the signal to pass between the two LTP valves.
I believe the principle of 100R cathode node bleeder is intended to allow any error signal generated by the LTP to escape. Without it any error is integrated back into the signal via the valve cathode acting as an input (as intended in a the operation of a LTP). The value of 100R is just one suggested by Allen Wright in his PP1C power amp. In my previous power amps the issue of stridency has not arisen and so I have always stuck with a 1Meg bleeder (this is essential to their stable long term operation of these designs).

I hope that long and converluted answer helps clarify. Essentially the Current mirror ensures DC current balance in the transformer but doesn't set the current. Its advantage over a CCS is its lower drop out voltage. The caps are essential to allow the signal to pass.

Shoog
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Old 6th July 2012, 01:40 PM   #67
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Thank you Shoog, it's not convoluted at all but very clear,particularly as I had not even heard of a Wilson mirror a few days ago and will be very pleased to add this to my repertoire, so to speak. The details of the cap junction bypass will require some reading to understand the theory in detail but I am happy simply to take advice for now.
One detail though; the collector of the right hand transistor is clamped at 2 diodes drops above ground but the voltage of the upper left hand transistor's collector is only set by it's load resistor which at present will be about 2.5 volts. Is this your intention or is it a misprint in your latest schematic?
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Old 6th July 2012, 02:38 PM   #68
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I am in sort of the same situation - I only discovered this current mirror out of necessity and am only just getting my head around its internal workings. You are correct in saying that R2 & R3 should be equal, but its not that critical since you could remove R3 altogether and it would still work.

Having just measured it - the single transistor is actually loaded with a 62R resistor followed by a 100R resistor on the valve pin and measures 2.32V at the top of the 62R resistor. The twin transistor leg is missing the 62R resistor and measures 2.3V also - so it is compensating internally.

I was thinking about that myself only moments ago and realised my original intention (to keep dissipation in all transistors equal) isn't possible in the Wilson Current mirror. I will probably equalise them to see if it make any difference.

Shoog

Last edited by Shoog; 6th July 2012 at 02:54 PM.
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