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DoomPixie 7th February 2006 02:41 PM

Hybrid amplifier help...
2 Attachment(s)
I have been experimenting with a hybrid amplifier design, After tragically loosing all the magic smoke a couple of times i decided to switch the positive and negative rails and now the magic smoke escapes no more (still think the positive and negative are the wrong way round now though but it works.. ) and i have output! not exactly good quality.. not even sure it could be classed as music.. but it is there.. and loud... it sounds like the amp is clipping or the speaker is broken but the speaker is fine.

Schemnatics of the original are atached below, my amplifier is slightly different as i just used things i had lying around that were already built, i will try and get updated schematics posted asap!
Basically The SS output module i am useing apeared to only loose its magic smoke from one resistor BUT i am not 100% sure that the driver transistors arent damaged so i will test them next after i find out

The tube stage..
basically the 1K resistor is a 120 or 150 Ohm in mine because i simply used a circuit i had already built for something else
How does this affect it?
Could this be my problem?

I'm thinking mabey it is stoping the amp from geting enough voltage or something because i have to have the source cranked right up in order to get anythign that remotley resembles music out of it..
Thanks in advance,

DoomPixie 7th February 2006 02:48 PM

2 Attachment(s)
and this is basically the circuit as i have it running now..

anatech 7th February 2006 03:09 PM

Hi DoomPixie,
Your second diagram shows your Vbe multiplier is shorted out. Error?

The diode in the emitter of the Vbe multiplier should be removed (short). The input impedance is too low to the output stage. I would use newer output devices that are more linear. MJ15022 / MJ15023 would work well here. The newer MJ21193 / MJ21194 would do well also. I don't think those drivers can deliver the peak currents that might be required, they could still be used for the first transistors in a triple darlington config, or another setup as you choose. The 270K resistor should be 270 ohms?

Many hybrid designs show the fault of a low output stage impedance, increasing this will bring better results. Feedback will improve the sound (from experience). You don't need much, try 10 dB as a start.


Tube_Dude 7th February 2006 03:18 PM

Hi DoomPixie

The 47 uF output capacitor is to low for a 8 Ohms speaker , use 4.700 uF at last...

anatech 7th February 2006 03:23 PM

Nah, eliminate that caps and reference the output stage to ground. Use a relay to short the tube signal to ground until the tube circuit settles down when turned on.


DoomPixie 7th February 2006 03:39 PM

2 Attachment(s)
the output cap is doing ok.. the 270k is infact 270ohm.. my mistake..
the output stage isnt the same.. there is no diode itn eh output stage on mine.. schematics of the output stage are enclosed.. i changed the 2SA984 for a BC327.. think it was a 327.. it works so much better after changeing that transistor.. the midrange is soo sweet on the amp, there is just something really wrong somewhere and the bass and trebble are all messed up.. sounds almost like a bad connection in a lead does when you shake the lead around now.. on and off.. except the midrange which is perfect..
anywah, here is the schematic for the output stage. :) the 330 ohm in that pic is 270ohm.. had to change it after if released its magic smoek and didnt have any 330ohm resistors :(

Tube_Dude 7th February 2006 03:41 PM


Originally posted by anatech
Nah, eliminate that caps and reference the output stage to ground.
In the case of direct connection to the load , it is wise to use some zeners in the feeding of the voltage rails for the 15k /12k resistors , feeding the VBE , if not , the offset voltage will vary with the rails voltage , that are also dependent of the mains voltage fluctuations ...

anatech 7th February 2006 03:49 PM

Hi Owen,
I suspect the tube stage is going crosseyed :xeye: trying to drive the output stage. Things will improve if you can increase the impedance seen by your tube stage.

I have build a few, if you reference the output stage to ground it will behave quite well. Not the same circuit, but this one has problems to solve anyway. If you redo the input to this circuit, you can lose the dependance on the rail voltages. CCS drive comes to mind for more than one reason.


DoomPixie 7th February 2006 04:13 PM

please slap me!
i never noticed that 15K resistor before....
I have been puzzeling over why the thing sounds better when i disconect the 12K resistor from the supply for ages! thats probably all thats and theres me changeing resistors and transistors and stuff trying to work it out and gradually getting better and better sound out of it... i'll go and add that now and see if it works. lol

DoomPixie 7th February 2006 04:29 PM

ok, i added that resistor and it got rid of the annoying problem with dc offset.. :) i was thinking something along those lines myself, the amplifier is behaveing very well at max volume now. but when i turn it down it sounds liek the tube isnt giving enough current for the SS section to switch properly.. How would i go abotu fixing this? The driver transistors are now BC327-25 and BC337-25.. changeing to these gave a pretty big improvement over the originals which i tried to measure useing hfe checker on my multimeter but the legs were too short so i just changed them for some transistors i had on hand..
So yeh, how would i go about increasing the impedance? :S also will that resistor in the tube stage make a diffrence? as i said it is meant to be a 1K but i am useing a 150Ohm still?

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