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Old 14th February 2003, 04:55 AM   #1
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<img src=http://home.pacifier.com/~gpimm/phono_front1-1.jpg>
Ok, Ok, I get the hint... I spent a couple of hours this evening and finished up the schematic and got the pictures in order for the phono pre. The values in the EQ network might change slightly after I get the reverse EQ from Hagerman tech.

Look in the schematics section. shematics and pictures are both there. Boy, it's time to update the sorry mess that I call a webpage!

Gary
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Old 14th February 2003, 07:52 AM   #2
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Holy Cow!!!!

I did not know you could work neat as well. (David)

Beautiful craftmanship!

Inspiring.

Cheers,
Bas
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Old 14th February 2003, 03:38 PM   #3
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Very nice preamp Gary and hope it measures to your expectations. Please let us know about the percived sound when testing with your audio system.

Thanks for sharing.
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Old 14th February 2003, 10:45 PM   #4
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Hiya Gary,

I just went and took another look at your site (I was visitor 3 after you posted the BBMCCS) and I must say I'm impressed with your phono pre. They look great, both physically and technically, and I'm especially impressed with the neatness of the layout, and those neato terminal strips (Tek?). Excellent!

Recently I was going to email you about the noise performance of your BBMCCS because I'd considered using it to load the tails in my differential MC phono stage (taken to a neg supply), and as anode loads, similar to what you've done in a phono stage for a Decca. It seems the latter part has been answered, but do you have an opinion on the first?

The David looks really interesting too. Lets see, I have all the tubes, about 50 spare lithium batteries.....hmmm.
What are the OPT's you're using?

Thanks again for all the great CCS's you've designed.
Regards
Brett

PS: The David and Phono schematics are up on Gary's page for those interested.
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Old 15th February 2003, 02:46 AM   #5
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Boy, I guess that I should have started a new thread instead of inadvertently hijacking Joels thread. Can we clip the end of this thread off starting with the picture of the pre and use it to start an new thread?

Yep, those are old Tek ceramic strips. The really old ones with 2-56 threaded posts comming out of the bottom of them.

I've only done one experiment with a dif amp so far. It seems like any noise that the CCS would contribute could be treated as a common mode signal and should cancel out.

I have tried a CCS loaded dif amp with CCS's on the anodes too. Only way to get the signal out was the mu folower outputs as the plate impedance was through the roof. That is definatly not ready for prime time.

Having had time to think about it for a month now I have a couple of things to try that might stabilize the circuit. With the CCS's on the anodes I think that a load resistor hung between the anodes of the pair might work good. This would give the tubes a differential load to work into and the mu folower action of the CCS's would buffer the diff amp from the outside world. Still might not be worth the effort though.

The David uses... Radio Shack 70 volt line matching transformers for outputs. With the power level maxed out at 1 watt the 10 watt tranny works pretty good. Low signal frequency response at .01 and .1 watts was in the low 20's to 30K. At 1 watt it was ~30 to 22K. The sound quality from the little things is really quite good! Exelent tight bass, clean mids, and smooth highs. Not in the same leage as my 300B amp but better than a whole lot of things I've heard. They sound much cleaner than the Wave 8's to my ear. The only down side is you must have high sensitivity speakers to let them sing.

Thanks for the compliments!

Gary
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Old 15th February 2003, 08:40 AM   #6
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Hi Gary,

Very nice looking construction there!

Just wanted to explore CCS loading of a diff pair; if we accept that the anode loads need to be equal for balance, and 2 infinities aren't necessarily the same, how about shunting each CCS with a high valued resistor, say a few hundred K?

In this way we effectively create a diff pair with very high matched loads without the need for an enormous B+. You lose the mu-follower output (so output Z = internal res of valve) and need to rethink the value of the CCS in the cathode circuit for the wee bit of extra current coming through the shunt resistors, but otherwise...

I'd imagine this kind of technique would work well for split load phase splitters too.

And I think it might help stabilise the op points for direct coupled diff pairs as you're fixing the current in every possible place.

Cheers
cv
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Old 15th February 2003, 05:03 PM   #7
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Finally found the page on the phono amp. You really should reorganize!

Looks great!

Only comment I can think of: 6GK5 has no bias. As a result, its zero-signal plate current will be very high, dropping with any application of signal (due to the rectification by the grid producing a negative bias voltage).
Lesse, since it's CCS loaded, plate voltage instead will drop significantly, changing the operating point.
When signal is applied, plate voltage will shoot up as bias is applied.
I guess at least your coupling caps are getting a workout...

CV: isn't that what grid leak resistors are for?

Tim
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Old 16th February 2003, 02:54 AM   #8
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The 6GK5 is setup with grid leak biasing. The grid leak resistor is 22Meg and provides about -.75 volts bias. The plate voltage of the stage is ~90 volts.

I have plugged in every 6GK5 I have and the plate voltage variation was less than 10 volts through 7 samples.

In stages that deal with smaller voltages and have relativly low impedances driving them grid leak works well. No cathode resistor and/or bypass cap to deal with.

Gary

P.S. Sounds great too!
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Old 16th February 2003, 08:11 PM   #9
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Hi Tim,

>>>CV: isn't that what grid leak resistors are for?

Well, in a cap-coupled amp, they'd certainly balance the (CCS loaded) diff amp outputs at ac, but I think we'd still need shunts across the anode CCSs to stablise the dc conditions.

If the CCSs were all perfect, you'd have to ensure the cathode CCS current was *exactly* equal to the sum of the anode currents. The shunt resistors give us some margin for error.

cheers
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Old 16th February 2003, 09:28 PM   #10
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On the topic of active loaded diff amps...

I forgot to mention above that the active loaded diff amp test setup has a variation of the battery biased CCS that provides good DC stability. Using high value resistors could be an excelent way to balance the AC performance. Using the active loads you could adjust the AC balance to be spot on at 100Khz just by moving wires around. Too sensitive to stray C. The resistors might be the answer.

The variation consists of removing all the batteries, then setting up a 3 resistor string to establish the bias. In this manor the CCS current varies with the DC voltage across the CCS. The AC performance is similar to the battery biased version and acts as a CCS down to frequencies in the low single digit numbers. At DC the CCS has an impedance that is only in the mid Kohm range.

The example shown is about 30K at DC and a couple of hundred meg at audio frequencies. It is setup to source 5ma when 220V DC is across the CCS.

The original purpose for C2 in the earlier CCS's was to capture the drain-gate capacitance leakage of the upper mosfet. In this version it still does that function and in addition captures the AC current from R7 caused by the signal voltage. By routing the 2 sources of AC leakage current to the source of the lower mosfet it is added back into the summing node of the CCS and keeps the effective shunt capacitance low. Here is a schematic of the voltage variable CCS.
<img src=http://home.pacifier.com/~gpimm/vvccs.gif>

Gary
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