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Old 14th April 2015, 07:53 PM   #1
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Friends, very good morning. I have determined to make upgrade my amp Rotel RA-970BX. I am replacing power capacitors; in improving the rectifier bridge by a high-speed and higher current.
But I read on the internet I found this article on the quiescent current.
Adjusting the Bias Of Your Amp [English]
It's been a couple of years I've gotten thank you improve the offset voltage + -20 mv (before was about 70 + -68 mv.
I would like to advise me about the article I mention them.
I'm a little afraid increase quiescent current without consulting the results.
This article speaking out at 50-100 mA, with the manual Rotel's 8 mv (about 36 mA).
Greetings and hope your answers.

Last edited by ALPUY; 14th April 2015 at 08:40 PM.
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Old 14th April 2015, 08:28 PM   #2
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100mA? A good way to cook your amplifier!
Always use the specified measurements. They are what they are for a good reason.
You cannot improve a class B amp by trying to run it in class A. The heat sinks are not designed for it and you may stress the semiconductors
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Old 15th April 2015, 11:26 AM   #3
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I found my amp running 150 ma idle bias current on one channel, with the output transistors, NTE60's , fine. The secret was I had put fans on the totally inadequate heatsinks. The cause was a mis-matched transistor pair sensing the feedback in a closed loop feedback control circuit. The transistor pair had not been mismatched when I installed them: the circuit worked properly controlling bias current to 20 ma 18 hours a day for some period after I built it. Perhaps SOA violation on the 2n3904 transistors overstressed them. Neither sense transistor was shorted on either junction. So the bias control circuit went open loop. I ran this amp 18 hours a day for a couple of years after I put the closed loop bias circuit modification on it. The sound was excellent. I only measured the bias because I kicked some wires loose putting records away and had to repair it. The torn off wires were on the other channel. I had already destroyed the sense transistors and replaced them with something tougher on the other channel. That 2n3904 failed because the wire to the transistor case did not make contact due to some heat sink grease on the screw, putting 72 volt on the sense transistor and shorting it. The replacement transistors were ztx653 with Vceo of 100v.
1/8" thick aluminum with no fins or holes is not much of a heatsink. But a PCAT power supply fan blowing right on it covers a lot of sins. That is a 3 1/2" fan. I run the amp fans at 9 v instead of 12v for less noise. That is one fan for each channel, a really obsolete layout of heat sinks: 1966 design.
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Last edited by indianajo; 15th April 2015 at 11:50 AM.
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Old 15th April 2015, 03:02 PM   #4
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Hello friends. My question is whether it can cause damage to the power transistors in the output by increasing the bias current of 36 mA which currently owns (the amplifier is powered with + 49 to -49 volts. If not mistaken 98 x 0.036 = 3.6 watts transistor at rest. As has 8 output transistors, 4 per channel, would dissipate 28.8 watts at rest).
My question is, if the amplifier can deliver 90 + 90 watts into 4 ohms may damage the transistors if doubling eg quiescent current, or take from 36 mA to 60-70 mA ?. If we took him to power would rest about 50-60 watts. (my reasoning is: if the amplifier supports maximum 180 watt rms, because it will not stand 60-70 watts at rest?).
If my calculations are wrong thank me say it.
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Old 15th April 2015, 03:44 PM   #5
ilimzn is offline ilimzn  Croatia
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Your reasoning would go out of the window if you tried to PERMANENTLY expect any power up to the rated one from your amp. It is NOT made for it and unfortuntely few understand that. Try half the rated power into nominal load and most commercial amplifiers (and lots of boutique ones too) will overheat with rather bad consequences. This is because the amp has to be constructed for peak power in order to cater for signal peaks, not a constant average, which is perhaps 10% of the peak.
Increasing the standing current will increase the steady state temperature of the heatsink. This leaves LESS of an increase before a dangerous level is reached, to cope with the real average power. In other words, any standing current ncrease adds a constant to the dissipation. THe maximum power, voltage and current conditions for a transistor in a real situation are all derated for temperature. In other words, higher standing temperature = less headroom for the transistors to work in.

Finally, PLEASE forget about that article, which periodically turns up referenced here and actually do some research on optimal standing current, and in general temperature of semiconductor operation and heatsink dimensioning. Your init, just like any other commercial unit, is engineered for the recomended standing current, BARELY. THat article is probably responsible for a lot of 'audiophile tweaks' gone bad, and I REALLY wish everyone who had that happen, to send the bill to the author, who is named at the end of the article. I hope that will teach this person the meaning of the word 'caveat'.
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Old 15th April 2015, 04:06 PM   #6
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Dear iliman, I appreciate your opinion. It has really made me think his advice. And he is right in advising me.
As much I love my Rotel, I'll leave as indicated by the manufacturer with the quiescent current contained in the technical manual. Anyway, sounds great.
I understand that if I want a Class A amplifier, should I invest in buying it.
Greetings and thanks to all who responded to my question me.
Alpuy.
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Old 15th April 2015, 07:44 PM   #7
ilimzn is offline ilimzn  Croatia
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If you want to exoperiment, you can do so very carefully but keep in mind that it will need careful monitoring of the heatsink temperature - especially considering that summer months are coming when ambient temperature is higher, and with cooling, it's all about the difference between ambient and heatsink tempearture. So, when ambient goes 10deg higher, so will the heatsink - in fact it may even go higher than that a bit due to various other effects to do with turbulent airflow. What makes thing more compliated is that taking the cover off also changes things, so does using the amp in a non-horisontal position (always avoid it unless it's specifically stated in the service manual that it can work that way, for example some amps can work bottom side up for bias and offset adjustments).
In the very short term, there should not be much of a problem and it may also turn out that slight adjustments produce the effect you want (sometimes even lowering the current!). But long term, you should be careful and keep in mind that the manufacturer has to design for a wide range of conditions of operation, so whatever they chose to set the standing current to, is not just a guess. That being said, a 10% variation should fall into normal tolerances and you should be safe within those limits.

Last edited by ilimzn; 15th April 2015 at 07:49 PM.
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Old 15th April 2015, 08:21 PM   #8
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beyond thermals notice that distortion figures may alter with other bias than specified

It is supposed that the manufacturer already performed a number of tests to define proper bias which will be a balance between thermal stability and quality of sound

Farther more please notice that Class A amplifiers are not suitable for all type of music reproduction ...There are types of music the actually perform better with typical class AB bias of the region of 50-100ma rather an overbiased Class AB amplifier of the region of 150-500 ma .

Kind regards
Sakis
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Old 15th April 2015, 08:31 PM   #9
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iliman, following your advice, I got the cover of the amplifier for measuring volts at the intended to calibrate the bias. The manufacturer provides 8 mV (36 mA).
I have seen that after two years had already been modified to 12mV in L and 9 mV in R.
I have put in 8 mV almost about to change to 9 mV even for both channels. I have found that after one hour remained the same and that the heat sink was hot to the touch.
For the health of Rotel better leave it as is, I repeat sounds great. Thank you all for your opinions.
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Old 15th April 2015, 08:34 PM   #10
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east electronics, thank you too. You were writing while I answered ilimzn. Thanks also.
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