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Hybrid amplifier.. Please help..
Hybrid amplifier.. Please help..
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Old 1st March 2006, 05:40 PM   #71
anatech is offline anatech  Canada
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Hybrid amplifier.. Please help..
Hi Owen,
Don't worry one bit about 30mV. Ignore that completely.

Don't design the PCB yet, bias is fine as long as it's controlling the bias current. You still need to play with high level performance (extra stage) and CCS or bootstrap. Take your time.

Once you are done, you can design the board and troubleshoot that.

The relay between the tube stage and bipolar stage is a must. Simple timer circuit. Just time how long it takes on average for the plate voltages to settle in and add a bit of time.

-Chris
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Old 1st March 2006, 05:41 PM   #72
anatech is offline anatech  Canada
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Hybrid amplifier.. Please help..
Gee Owen,
We were both posting at the same time.

-Chris
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Old 1st March 2006, 05:52 PM   #73
AAK is offline AAK  United States
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A relay on the output will do. Owens design is similar to mine. But I use a servo to keep the output dc offset under control. Here's a page of my site, with some pics of the hybrid amp that's still under development.

http://www.alsaudiokits.com/hybridampkit.html

Cheers,

Al
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Old 1st March 2006, 05:58 PM   #74
anatech is offline anatech  Canada
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Hybrid amplifier.. Please help..
Hi Al,
Why the devil would you subject the BJT's to that voltage spike? It's 100% avoidable.

I have working hybrid designs as well. Mine do not require an output relay, except for protection purposes maybe. There is the slightest of clicks with your ear to the speaker at turn on with only the tube stage to output relay.

-Chris

Edit: Al, there is only an ad for your amp on your link, no circuit details. The link is therefore not helpful except as an ad.
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Old 1st March 2006, 06:01 PM   #75
DoomPixie is offline DoomPixie  Wales
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Hi AAK, I cant really take credit for the design, If it wasnt for Everyone here I would never have got it working. But I am glad i stuck at it, Now that i know silicon dosn't hate me as much as i thought it did i feel more confident and am looking forward to improveing the design further. At the moment though i want to build it like it is so that i can listen to some music through it for a few weeks and get used to it and then i will be more able to tell if the changes i make to the design ont he next one i make have made any improvements over this design, I have a feelign that when i have sorted all the cabeling out and have set the bias up properly it will sound even better than it does now!
Many thanks,
Owen
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Old 1st March 2006, 06:03 PM   #76
DoomPixie is offline DoomPixie  Wales
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there we go again chris.. .posting at the same time.. lol
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Old 1st March 2006, 06:06 PM   #77
anatech is offline anatech  Canada
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Hybrid amplifier.. Please help..
Hi Owen,
That'll teach us!
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Old 1st March 2006, 07:09 PM   #78
DoomPixie is offline DoomPixie  Wales
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I found another bug... if i set the bias higher than 8mV (accross both resistors) it slowly but surley keeps riseing all by itself i have checked that the output transistors and Q3 are makeing proper contact with the heatsink and thats ok.. they dont measure atall above room temperature so cant be heat related anyway as far as im aware? :S mabey i need to move the 10uF cap closer to the bias transistor? or is this totally unrelated? i have just been listening to it pretty much since my last post at 8mV with the bootstrap caps in place and it didnt move a bit.. and sounded pretty good too.. no real difference in soud with and without bootstrap caps at this bias setting but i cant safely get the bias higher at the moment to test it at higher bias settings.. any ideas?
Thanks,
Owen
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Old 1st March 2006, 09:51 PM   #79
anatech is offline anatech  Canada
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Hybrid amplifier.. Please help..
Hi Owen,
Mount the drivers to the heatsink as well. You may see the bias rise but it will (or should) start to come back down after a while.

If you see the bias start to rise, and the rate is accelerating, that's thermal runaway. Not your friend. Power down.

-Chris
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Old 1st March 2006, 10:14 PM   #80
ilimzn is offline ilimzn  Croatia
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Quote:
Originally posted by DoomPixie
I found another bug... if i set the bias higher than 8mV (accross both resistors) it slowly but surley keeps riseing all by itself
Looks like thermal instability. Could be runaway, in that case like Chris said, put drivers on heatsink as well.
It is normal that the bias starts rising on it's own though it should stabilize or even go down somewhat long term. Make sure Q3 is in good thermal contact with the heatsink as well (proper thermal grease used and all).

Quote:
i have checked that the output transistors and Q3 are makeing proper contact with the heatsink and thats ok.. they dont measure at all above room temperature so cant be heat related anyway as far as im aware?
Actually, it is, because the internals of the transistors heat up quicker than the heatsink, it takes a while for the temperatures to equalize (if you will, it takes a while for the heat to get from the drivers/outputs to Q3 via the heatsink).
To test this you can connect your voltmeter across the 0.33ohm resistors, and slowly add bias to get into the zone where it rises on it's own, say 10mV. Then leave the adjustment alone, keep your finger on the power switch, and just watch. If the voltage ever goes over say 120mV (this would be about 200mA bias current), switch it off. It is safe to say you have a thermal runaway problem. In the circuit you are using, as Chris mentione, it is likely because your driver transistors are not on the same heatsink, so they heat up 'without Q3 knowing about it' so to say. The cure is to put them onto the heatsink with Q3.
If you can get the current to stabilize, then you can go on and push the bias up till it settles at 15-30mV as mentioned before. It is fairly normal to see fluctuation when the amp is switched on and the heatsink is cold. There is a trick to get the process quicker so you don't have to wait (but you have to be careful with that power switch because if there is runaway, the current grows exponentially and it may become TOO quick!), and that is to use a hair dryer to heat up the heatsink (seriously!).

Quote:
I have just been listening to it pretty much since my last post at 8mV with the bootstrap caps in place and it didnt move a bit.. and sounded pretty good too.. no real difference in soud with and without bootstrap caps
The bootstrapp will show it's advantages at higher volume (or lower sensitivity speakers), and complex loads where the tube section may run out of current. It would help if you could post a schematic for your tube stage.
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