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Old 10th April 2008, 04:47 AM   #431
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All,

Here is my newest "Baby Huey" monoblock. Power supply not shown.
The source followers I believe make the amp sound much more powerful. They act (in combination with fixed bias) to eliminate any blocking distortion. It also isolates the load on the diffamp and will allow the shunt feedback to be slightly more effective.

The 350V rail means that the thig develops nearly 12 Watts - I was using Hammond 1608 (10 Watt rated) Output transformers but core saturaion was evident at anything below about 35Hz. Changing to the Vintage A&R transformers (core about 50% larger) made the amp sound cleaner and more powerful. The A&R also had a better top end. Having said that the Hammond 1608 sounded very respectable. With a 300 or 320V HV rail I prbably would have left the Hammond in. With 350V I'm developing just a little bit too much power for it and the bigger core tranny helped.

The power supply is not shown.
+350V comes from a 285-0-285 @100mA secondary. 3A Ultrafast diodes with 10nF 3kV ceramics across them, into 5uF/900V polyproylene, thru a 2H 200mA choke to a 470uF/500V electrolytic with another 5uF polyproylene across the electrolytic. (Note I originally had a Hammond 8H 100mA choke here, going to the 2H 200mA choke I though cleared up a little "muddiness")

+75V comes from the +350V. 68K/2W to a 75V 1.5W zener, 22uF/100V and 2u2 100V poly propylene filter caps across the zener. A 30V or 36V zener would do. You just want to keep at least +25V on the drains of the MOSFETs. 75V Zener is just what I happened to have in my parts bin.

EL84 heaters off a 6.3V @1.5A winding

Not enough currentt capability to also run the 12AX7 heater BUT there was a 5V @2A winding. I bridge rectified that using Schottky Diodes and a 10,000uF to get 5.95V DC. Close enough. I connected the -ve side of that to the +75V rail. This reduced noise a little.

The negative (-45V) rail uses a separate transformer. A 18-0-18 @ 1A. I left the CT unconnected and used Ultrafast Soft recovery diode bridge across the full winding. That gave me 52 Volts. I inserted a 220R resistor between the diode bridge and the filter cap to drop that to 45V so that I could use a 470uF/50V Blackgate electrolytic which I had on hand. The series resistor and the Blackgate both help to keep that -45 rail squeeky clean.

Cheers,
Ian
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Old 10th April 2008, 05:11 AM   #432
AKSA is offline AKSA  Australia
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Impressive, Ian. Thank you for posting.

Do you think that constant current sources on the sources of the mosfets would reduce Vgs distortion further? Would it be worth it? The way I see it there is a chance this could decrease odd order distortion, and slightly lessen compressive distortion at higher output.

Really like the tiny cap coupling on the mosfet gates, and the direct source coupling to the EL84 grids. Wonderful design!

Thoughts?
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Old 10th April 2008, 05:21 AM   #433
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Hugh,
I'm almost certain that CCS loading of the MOSFET Source Followers would give some benefits. Whether it would be noticable or not - who knows. As you would apreciate there comes a point where the "simple design" suddenly is not so simple any more and you have to decide whether to finese the design for ever or to draw a line in the sand and say that is it. I've basically done that with this design. Its providing lovely music while I get on with the next project. A total braek from "Baby Huey". I've had some Plitron PAT 4006 Toroidal Output trannies (Raa 1885:5 Ohms, 100W, full power bandwidth 27Hz to 235kHz) sitting on the shelf for about 4 years. I've ordered a couple of quads of 300B. So I'm in the pencil and paper stage of a couple of parallel push pull 300B monoblocks. They (I hope) will be my next step up in the audio nivarna quest.

And my reference amps - Your AKSA Lifeforce 55, naturally.

Cheers,
Ian
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Old 10th April 2008, 05:47 AM   #434
Yvesm is offline Yvesm  France
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Hi Ian,

Quote:
Originally posted by gingertube
. . .
As you would apreciate there comes a point where the "simple design" suddenly is not so simple any more and you have to decide whether to finese the design for ever or to draw a line in the sand and say that is it.
Ian
Your last implementation opens a wide door to paralleling output tubes !

Great.

Cheers, Yves.
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Old 10th April 2008, 05:49 AM   #435
AKSA is offline AKSA  Australia
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Hi Ian,

Another thought....

The Zout of the mosfets is so low that you could actually indulge a little RF design here. You could ground the grids of the EL84s, and drive the tubes from the cathodes!!

As you know, this increases bandwidth and speed, and reduces distortion further still. AND, it would be unique.....

Say, my LF55 the reference!! That's really something, thank you!

Cheers,

Hugh
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Old 11th April 2008, 01:40 AM   #436
jueic is offline jueic  Taiwan
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Default Baby-Huey Monoblock

Hi, Ian

Another impressive circuit !! thanks..

Two things.

1. the adjusting pot connecting CCS and 12ax7 cathode is gone.. I really like the adjusting pot which helps to produce sharp music image.

2. the B+ now is 350V..about 15% above the EL84's 300V spec. Likely is ok judging from my Dynaco sca-35 (now my baby-huey) was running even higher voltage sometimes.

Cheers

JueiC
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Old 11th April 2008, 01:51 AM   #437
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The "Balance" adjustment pot can be put back in and if you are using modern production ECC803S or 12AX7A where the two triodes are not necessarily well matched it is probably a good idea to do so.

For this build I used selected NOS Philips 12AX7A where the two triodes were matched perfectly and so did'nt need the balancing function - BUT I am now limited to using selected 12AX7s.

The JJ EL84 I'm using cope with 350V OK although you may note I pushed the screen resistors up from 33R to 150R as a precaution.
350V is just what I ended up getting with the power transformers I had. 300 to 320V would be better and a Hammond 1608 Output Tranny would cope better (it drops output power back to wiyhin the 10W rating).

Cheers,
Ian
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Old 11th April 2008, 02:58 PM   #438
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Ian,

I've just finished the original baby huey for about 3 weeks and enjoy very much with it. Great design for a newbie like me. But, all in a sudden you have another design coming up with fixed bias. Oh!!! it makes me ichy and eager to try, since the first one is so good and I suppose the fixed bias should be much better.

1). Can you tell me how you find ( sound wise ) between the two.

2). That two 50k pot is just for adjusting -12v for the G1 of the EL84. Am I right?

3). The test point showed at the cathode of both EL84, Is it just the check point for checking the 'Voltage', and or we have to make some adjustment here? and how?

4). Do we need any heatsink for the mosfet?

I know you like this design very much and I still believe you'd make some kind of tweaks so to improve it's performance. Should I wait a bit until you have the performance maximized ?

Thanks

Albert
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Old 14th April 2008, 03:31 AM   #439
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Albert,
The fixed bias version is a little bit better when driven hard - this is because it doesn't suffer from blocking distortion which is a problem with most cathode biased amps.

The test points shown in the new schematic are to measure the voltage across the 10 Ohm "current sense" resistors in the EL84 cathodes. You then adjust the 50K pots (with no input signal) to get 0.4 Volts at the test points - which means that you have 40mA of idle current. The voltages at the mosfet cathodes will be approximately -12V (between -10 and -14 volts).

ALL - Mods to the new schematic.
Extended listening showed that the amps were a little "slow" with respect to PRAT (Pace, Rhythm and Attack). Dropping the feedback resistor from 20K back to 15K (the value used in the original) fixed that.

A quick calc of the coupling capacitors. 10nF into 1M2 gives a low frequency corner at 13.3 Hz. I decided to double the caps to 22nF to shift this down to 6Hz. I don't know if I really noticed any difference in the sound to be perfectly honest. Theory says it should be better with 22nF caps instead of 10nF but the output tranny low frequency response is probably preventing me from noting any real difference.
So if building from scratch use the 22nF. If you've already built it with 10nF I don't think its worth changing.

Cheers,
Ian
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Old 14th April 2008, 03:48 AM   #440
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Ian,

I'd certainly build a new one with your new schema, but I'd like to enjoy this baby a bit longer so to gather all parts to start the new one.
Thanks

Albert
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