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Bias on a Studiomaster 700e |
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Randy Bohannon 
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Joined: 29 May 2022 Location: London Status: Offline Points: 121 |
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 Topic: Bias on a Studiomaster 700ePosted: 25 February 2025 at 2:53pm |
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Hi all,
I’ve acquired a working one of these to match my busted 2000e and to prove to help me fix the 2000e modules that have so far defied repair. The 700e works well and passes a 20Khz sine wave at full power without showing a crossover glitches on the scope but measuring across the emitter resistors at idle I get 0v. There is a provision for a bias pot on these and it’s marked in the schematic as “not fitted”, so would be easy enough to add. What sort of value might be appropriate for the bias figure, the generic class B 26mV across the emitter resistor? The other issue is the DC offset, 120mV on one channel and 160m? On the other, is that normal for these amps? Cheers, Edited to add schematic Edited by Randy Bohannon - 25 February 2025 at 3:08pm |
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kedwardsleisure
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Joined: 20 January 2009 Location: Staffordshire Status: Offline Points: 5157 |
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Posted: 25 February 2025 at 7:28pm |
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bias is automatic on these, if you're not getting a bit of idle current then something's wrong.
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Kevin
North Staffordshire |
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Randy Bohannon
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Posted: 25 February 2025 at 9:36pm |
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Thanks, no bias on either channel at idle, climbs to around 0.42V at clip on a 4 ohm load. I’ll pull and check the bias transistor.
Does the offset voltage seem about right? Edited by Randy Bohannon - 25 February 2025 at 9:37pm |
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kedwardsleisure
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Posted: 25 February 2025 at 10:40pm |
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a bit high if you ask me but never really noticed if it doesn't pop at turn on. I'd have rather seen 10mV or less.
Can't really measure bias with the amp running a signal  PS If the amp has blown up and been repaired I would check all the BAV20 diodes and zeners in the bias circuit as they can develop VERY small leaks that upset the dc conditions..same with the limiting transistors. Edited by kedwardsleisure - 25 February 2025 at 10:48pm |
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Kevin
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EmPro
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Posted: 08 March 2025 at 1:49pm |
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Just replace the 3.3v zenners with 2.7v and you're out of problems mate.
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EmPro Audio
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Randy Bohannon
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Posted: 19 March 2025 at 9:04am |
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Yes, sorry I meant that the bus was zero at idle but that the current through the emitter resistors tracked linearly with output perfectly, so physical issues with them or the output transistor joints etc.
I’ll check the bias transistor out of circuit, I’ve seen a case in a Cloud amp where the bus transistor passed a multimeter check but showed a gain of 2(!) under load. I do need to sort out the DC offset though, I would prefer it to a tenth of what I am seeing and Empro’s one has sub milivolt offset! For some reason the LTP transistors in my 2000E are not, or are no longer, very well balanced, relatively large differences in both gain and Vbe. I will pull the ones in this amp and see how well matched they are. I’m also tempted to remove the triac crowbar and fit a relay based speaker protection board as there is plenty of space, unlike a 2000e.
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EmPro
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Posted: 19 March 2025 at 12:17pm |
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To be honest i'll live the triac where it belongs because the relay will give you loads of trouble sooner or later and also reduces badly your damping factor,this kind of amplifiers have 20 fuses all over the place...so you're kinda safe even in the event of power transistors failure,the only reason you have high dc offset is just from differential input,semiconductors/cpacitors/resistors that are out of tolerances.
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EmPro Audio
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Randy Bohannon
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Posted: 19 March 2025 at 3:26pm |
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I hear you, but I was never a fan of crowbar DC protection circuits, they turn a minor mishap into a Chernobyl type blow out.
Damping factor… I take your point about the contact resistance lowering the damping factor but in the first instance it’s not that important with the 8 ohm speakers I will be using and secondly the click they make is very reassuring… as long one avoids the type where the signal passes through ferrous metal they don’t seem to add any distortion… I will have to think about a bit more about it.
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Randy Bohannon
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Posted: 19 March 2025 at 3:28pm |
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P.S. when my 2000e went last time, at full chat and four ohms bridge, it blew the centre leg clean of the triac! 😂
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EmPro
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Posted: 19 March 2025 at 4:40pm |
That amp was never ment to run bridge 4R or stereo 2R,so is kinda your fault mate
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EmPro Audio
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EmPro
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Posted: 19 March 2025 at 4:43pm |
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It was listed as 2R 1000wpc and bridge 4R BUT IF YOU DON'T PUSH IT TO HARD
so basically....neah....fully stabile 4R stereo and 8R bridge all day and night
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EmPro Audio
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Randy Bohannon
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Posted: 19 March 2025 at 5:05pm |
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lol, yes two pairs of 15022/23s do not have the SOA for 2r and 100v rails…in my defence i was young and it was one hell of a party…😂😂
I don’t know what they were thinking rating these 2r stable, they only changes from the non 2r stable 1600d seem to relaxed current limiting…
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Randy Bohannon wrote: