The 50E model is an early (dating from1966) solid state mono amplifier, before IC regs and other specialised power supply components were commonly available. It was designed primarily for utility applications in the broadcast industry, hence it's a one-channel or mono(phonic) amp. but there's no suggestion that means its a "mono block" quality design with all the associated hype. They were produced in large numbers for radio and TV sound studios, only mono then but there are still quite a few around in storage and private use. It preceded the stereo 303 domestic model which had much improved specs though by today's standards, the 303 isn't great anyway. There were dozens of various Quad 50 models deployed in almost every British and Commonwealth country's radio and TV studio, workshop etc. at one time but of course, there were also competing models from other broadcast industry contractors too.
The design requirements were different to domestic hi-fi stereo amplifiers. For studio and broadcast use (monophonic then) they needed to be bullet-proof, very reliable and flexible enough to connect with virtually any other audio equipment likely to be in a recording or live-to-air studio environment. The rear panel in/out connection strips, 600R input transformer option and multi-tapped output coupling transformer say it all.
The design requirements were different to domestic hi-fi stereo amplifiers. For studio and broadcast use (monophonic then) they needed to be bullet-proof, very reliable and flexible enough to connect with virtually any other audio equipment likely to be in a recording or live-to-air studio environment. The rear panel in/out connection strips, 600R input transformer option and multi-tapped output coupling transformer say it all.
Hi Ian, thanks for your explanation. I just got a pair of 50E to drive my Tannoy monitor gold, in my experience, it is good for vocal and string, and I very much enjoy it. the only problem is the 180-ohm resistor and 5.6V Zenor are extremely hot, I plan to install a TL783 instead of them.
The IC reg. is a bit drastic but given that it probably needs to dissipate about the same amount of heat, a small heatsink would be in order anyway. Wouldn't it be a lot simpler to just fit a 1W zener and up the resistor rating and size substantially? Those original parts have probably been working fine for many years, so the heat may have been unnerving but it hasn't really been a problem.
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You are right, a resistor and a Zener is the simplest circuit. But you know the power consumption on a 180-ohm resistor is 6.4Watts, and Zener 1.1Watt, causing high temperature. if replace an IC reg instead, the current only 14mA passes the reg on idle. the only reason that needs to be considered is how the IC impacts sound quality.
The high temperature hasn't been a problem though - it only offends our layman's sense of how things ought to be, inside the box. FWIW, just a standard 7805 or 78L05 with a signal diode in series with the reference leg and couple of caps, would give you a reasonably clean 5.6V with some capability in reserve.
Regarding sound quality, I'd venture that regulating the 5.6V supply to the driver transistors reduces any existing supply modulation and increasing the amount of regulation could well affect the sound. Whether it is perceptible will be down to the listener and likely, an expectancy effect though. So It would seem appropriate to use less regulation with a lower performance type of reg. like 78L05 there, rather than a high performance type if you wanted the least change in sound quality.
Regarding sound quality, I'd venture that regulating the 5.6V supply to the driver transistors reduces any existing supply modulation and increasing the amount of regulation could well affect the sound. Whether it is perceptible will be down to the listener and likely, an expectancy effect though. So It would seem appropriate to use less regulation with a lower performance type of reg. like 78L05 there, rather than a high performance type if you wanted the least change in sound quality.
Your suggestion is my thinking, the max input limit is 35V for a 7805, but 50E supplies 38V or more, so the 7805 does not suit the circuit. The TL783 is the best choice for this modification, and install a 100uF capacitor to the output terminal.
You're correct of course, about the reg's voltage limits - my bad. I still wouldn't be too concerned about the heat though. For others to see the area of concern, there's a good, clear pic of an original, well used amplifier board here: https://www.dadaelectronics.eu/uplo...ad-50E-and-D-Upgrade-Revision-manual-V1.3.pdf
Considering the likely total hours in use, the darkened areas around R32 aren't bad at all for an ancient, paper/phenolic type PCB.
Considering the likely total hours in use, the darkened areas around R32 aren't bad at all for an ancient, paper/phenolic type PCB.