The Cyber-Spy.Com Usenet Archive Feeds Directly
From The Open And Publicly Available Newsgroup
This Group And Thousands Of Others Are Available
On Most IS NNTP News Servers On Port 119.
Cyber-Spy.Com Is NOT Responsible For Any Topic,
Opinions Or Content Posted To This Or Any Other
Newsgroup. This Web Archive Of The Newsgroup And
Posts Are For Informational Purposes Only.
From: "Graham Holloway"
Subject: Re: Rebiasing a solid state amp
Date: Fri, 13 Dec 2002 09:00:42 -0000
NNTP-Posting-Date: Fri, 13 Dec 2002 09:01:09 +0000 (UTC)
X-Newsreader: Microsoft Outlook Express 6.00.2800.1106
The resistors are actually 0.2 ohm (the R represents a decimal point). I
suspect that normal quiescent for this type of amplifier would be around
50mA. Therefore about 10mV across each resistor.
To bias higher for low level class A operation you could try increasing the
bias (V4-Iq) for up to 200mA or so (40mV across each resistor) but this
should be done in small cautious steps, waiting for the thermal effects to
stabilise each time. On the other hand it might be the type of circuit that
uses the driver to produce a class A output at low levels. These tended to
produce a delayed form of crossover distortion. There is always the chance
that the design is a bad one that thermally. On the other hand it is still
working after 25 years.
Power will probably be over 100 watts per channel into 8 ohms. I can't speak
for its ability to operate into 4 ohms.
"Russellsit" wrote in message
> I have acquired an solid state amplifier. On the front panel it said:
> Monogram Professional Audio
> class a+
> studio reference
> handcrafted in England
> It is the usual rack-mount size width and would occupy one and a half
> rack spaces. The interior shows the power supply in the middle, and
> two amp modules at the sides. There are only 2 TO-3 transistors each
> side (2n3773). There is a sticker beneath the left circuit board with
> a date of 17th Sept, 1978, so this is a rather old amp. From the size
> of the heatsinks and the power supply (2 capacitors of 10000uF, 63V
> and a big EI transformer), to make it running in class A is not
> difficult when it was manufactured. I have also done some crude
> measurements on the amp. The power supply is +/- 65v. When there is no
> signal in, the voltage across the 20ohm resistors is not recordable (?
> 0 volt) (? class B operation).
> I have noticed that the TO-3 transistors do not feel hot. In fact it
> is only a little bit warm to touch, may be about the same as body
> temperature. The heatsinks are even cooler. In contrast, the drivers
> and the heatsinks are very hot. I found this very puzzling as the amp
> claimed to be class A+ but yet feel much cooler than my NAD 302. There
> is no infomation on the amp about the power output, but I guess it is
> at most 20w from the 2 transistors.
> I tend to believe that it was originally made to run in class A, but
> was somehow modified later to give a higher power output (class B
> now?). So I am looking into whether I can re-convert it back to class
> A (or at least class AB).
> On a closer look to the circuit boards, there are 4 labelled trimpots
> each side. They are
> V1 gain
> V2 +lim
> V3 -lim
> V4 Iq
> All of the resistors on the boards are low wattage type, except 2 next
> to V4. On the resistors I saw R20 5% CB5. I guess they are 20ohm
> resistors, may be 5 watts rated.
> I guess V4 is the control of the idle current. However, if the higher
> wattage resistors is for gauging the idle current, the maximum current
> through it will be limited to a small value by the 20ohm resistor. In
> addition, there are 2 of them each side, and I do not know on which
> one I should base my measurement.
> I would be most grateful if you could tell me how to do the rebiasing,
> or if there is any catch inside or something I need to be careful
> Thank you for your time in reading this long post.
> Best wishes,
> Russell SIT
Go Back To The Cyber-Spy.Com
Usenet Web Archive Index Of
The sci.electronics.design Newsgroup