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Old 3rd April 2011, 12:25 PM   #21
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Speaking of valves, I'm thinking about putting a valve pre in this amp (mostly just for the novelty) as a guitar amp, can you think of any problems with using across the entire secondary (~70Vpeak) doubler to get me up to about 140Vpeak?
Haha and yeah I'm 24, so those amps are probably before my time...
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Old 3rd April 2011, 01:51 PM   #22
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Quote:
Originally Posted by mnturner View Post
Speaking of valves, I'm thinking about putting a valve pre in this amp (mostly just for the novelty) as a guitar amp, can you think of any problems with using across the entire secondary (~70Vpeak) doubler to get me up to about 140Vpeak?
Haha and yeah I'm 24, so those amps are probably before my time...
Just thinking quickly... a doubler doesn't actually double the AC voltage as such, it gives an output equal to double the DC value a normal rectifier and cap would give. Helps if you see it drawn.
Voltage doubler - Wikipedia, the free encyclopedia


Grounds... the SS amp is on a split supply. The "ground" of the doubled output isn't the same point and they can't be tied together.

Why not just use a simple FET preamp to give the same type of harmonic distortion as a triode instead.
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Old 3rd April 2011, 02:18 PM   #23
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From what I've gathered (and used) the 2SC3503 or the KSC3503 make for good VAS transistors.
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Old 3rd April 2011, 03:04 PM   #24
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Yeah sorry maybe I wasn't clear; the transformer I'm using is about 25-0-25Vac, so it gives +/-35Vdc rails when rectified. If I used the double across the entire secondary winding (~50Vac which would give ~70Vdc rectified) I should get close to 140Vdc?
The grounding issue is one thing that definitely occured to me, but I don't necessarily see why it would be a problem. The low voltage at the bottom of the cathode resistor of the tube wouldn't be at the 'ground' voltage at the centre of the reservoir capacitors, but is that a problem? It would act as ground for the valve section of the circuit which would be cap coupled to the next SS section, so DC voltages are removed anyway. Does what I'm saying sound like it makes sense or am I missing something?
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Old 3rd April 2011, 05:20 PM   #25
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Best if you draw it out.

Cap coupling the audio is no problem. The grounds are. The SS uses the centre tap ground as an absolute reference for audio signals (input ground, feedback ground, speaker return). The valve stage will use what ? The lower end of the cathode resistor ? That would be the normal point that all the valve stages would use as their reference point for ground returns etc.

As the two grounds, SS and Valve are at different potentials what do you do ?
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Old 4th April 2011, 08:45 AM   #26
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I see what you mean now Mooly, simulated it and found that you were correct. You can use a doubler to produce two nodes with a constant doubled voltage across them, but they will be oscillating relative to the ground voltage of the rest of the circuit, so the quiescent voltage of the tube stage would also be oscillating relative to the ground voltage... Looks like I'd need a separate transformer or to just run the tube stage off the 70Vd between the +/- rails.
Thanks for pointing that out, I couldn't get my head around the concept until I simulated it.
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Old 12th May 2011, 04:00 AM   #27
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Default Blameless style amp redesign and build

Hopefully this isn't considered reviving an old thread. I took your advice with regards to Pdiss in the VAS and CCS transistors, and used better output devices in a 2channel rebuild of this project. The power supply is now a 35-0-35V, 300VA toroid giving about 51v rails unloaded, powering both channels. Power devices are mounted on a large fan assisted heatsink rated at 0.6degC/Watt without fan and 0.3degC/Watt with the fan. There are no signs of instability driving reactive loads (2uF cap across 8R resistor), and whilst I haven't yet accurately tested distortion I expect it to be small.
The amps are now putting out about 135W each into 8R, but are limited by the single slope VI limiting to about 150W into 4R purely because I didn't want to use two pairs of output devices per channel (although the boards do have provision for this) and the transformer is rated at 300VA.
The only things I dislike are the fan noise, which isn't a huge issue as I only turn on the fan when playing so loud the fan noise can't be heard, and the fact that in negative clipping some small oscillations are seen, I'm not exactly sure why?
Thoughts, comments, opinions welcome.
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Old 12th May 2011, 06:16 AM   #28
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Hard to say why there is scillation of clipping, and I assume you mean running with no load too... first thought is layout and feedback and the return points of the zobel nework etc. D3 and D4... IN41848 ? is that just a typo error.
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Old 12th May 2011, 06:20 AM   #29
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Yeah it was with no load. It looks like ringing in the high audio band only on the negative clip.
The return of the zobel has its own separate return to the star ground, as do the on board reservoir caps (100uF and 100nF) which I forgot to include in the schematic.
Feedback is taken from a separate tee off the main output track.
D3 and D4 are actually 3A 400V types, forgot to mention that.
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Old 13th May 2011, 01:24 PM   #30
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I ran some THD tests with ARTA software; I first directly looped the soundcard and tested THD with averaging to get a benchmark, then ran the loop through the amp into a passive buffer consisting of a pot and some clipping diodes to protect the input of the soundcard. The output was set to 8.9Vrms (10W into 8ohms) for most of the tests and the buffer was used to attenuate it to exactly the same output as the soundcard's output voltage (1Vpk-pk).
I then subtracted the THD found for the soundcard alone from the THD with the amp in the loop to get a measure of the amp's THD.

1kHz, Amp unloaded: THD: 0.0006%, THD+N: 0.0001%
1kHz, 8R Load (10W): THD: 0.0004%, THD+N: 0.0004%
1kHz, 4R Load (20W): THD: 0.0008%, THD+N: 0.0015%
2kHz, Amp unloaded: THD: 0.0005%, THD+N: 0.0001%
2kHz, 8R Load (10W): THD: 0.0003%, THD+N: 0.0004%
2kHz, 4R Load (20W): THD: 0.0002%, THD+N: 0.0015%
5kHz, unloaded: THD: 0.0008%, THD+N: 0.0002%
5kHz, 8R Load (10W): THD: 0.0005%, THD+N: 0.0003%
5kHz, 4R Load (20W): THD: 0.0002%, THD+N: 0.0003%
10kHz, unloaded: THD: 0.0001%, THD+N: 0.0001%
10kHz, 8R Load (10W): THD: 0.0034%, THD+N: 0.002%
10kHz, 4R Load (20W): THD: 0.0041%, THD+N: 0.0022%

Unloaded 28.3Vrms @1kHz: THD: 0.0003%, THD+N: 0.0001%

Obviously the results are pretty questionable; in many cases the THD+N is lower than the THD which I can't explain, and in many cases the THD into a 4R load is less than that into an 8R load, which could possibly be due to the amp being under-biased and the extra heat generated with the larger currents causes Vbias to come closer to the optimal value.
The results at 10kHz are particularly unreliable because my soundcard is on 16bit, so pretty much all harmonics after the second would be cut off.
Despite all this I'm happy with the results as they suggest the amp is producing less than 0.0009% THD, at least below 5kHz.

Any comments regarding the measurement method and results are welcome!
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