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1970s Design Indulgence

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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 13 Apr 2019 at 8:35pm
Other things left out from, or different to, the "test-bed" are:

1. The post fuse supply rail electrolytic is again a 4700uF snap-in (to get the maximum capacitance instead of the 2 x 1000uF 100V Philips caps on the test-bed).

2. The output stage "anti-parasitic" resistors (because I've never seen them in an output double like this).

3. A local 100uF supply rail decoupling capacitor to input ground (because I doubt it makes that much difference).

4. The bridge rectifiers have snubber capacitors this time.

5. The test-bed had a 100uF 100V bypass capacitor across the first 4700uF reservoir capacitor.

My hope is I can eliminate some if not all of these.

(by the way, this topic is also acting as my development diary, and that might be why it sounds like I'm obsessive?)
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 14 Apr 2019 at 7:42am
Couplingcapphobic?

The following is a quote from a designer who is what I'd call a reasonable and approachable fellow to get on with:

"Because these amplifiers are almost invariably considered "lo-fi" and will normally drive small speakers in horrible small plastic boxes, the coupling cap doesn't make much difference"

I doubt the Quad 303 could be classed as "lo-fi"? Rather than being used with small speakers in horrible small plastic boxes, it was/is often used with electrostatics, or conventional high quality speakers.

The 303 has output capacitors and so had several other well-respected 1970's amplifiers. In the case of the Quad 303 the output coupling capacitor value is 2,000 uF.

My use here of the humble output capacitor is in trying to get away from today's "click-bang" amplifiers. "Click-bang" was a phrase used by my colleague and old friend Paul from about 27 years ago. These days Paul is one of the guys responsible for making commercial radio transmitters work and knows one or two things about stability... at FM frequencies and above!

Almost invariably (to borrow the above phrase) DC coupled amplifiers are at risk of "click-bang", which as most amplifier designers will know (but never admit) is the sequence of switching the thing on, followed swiftly by it self-destructing.

Output protection circuits are often misused to prevent this condition, which is cheating by the way. And I have worked on the development of some amplifiers where they compound the problem.

To DC couple requires a level of complication which is just asking for trouble, and should the trouble occur "in the field" causes the expensive speakers such amplifiers are used with to have their bass driver voice coils melt! This is probably the most common dispute between customer and seller.

Can you imagine working in one room of a factory on a DC coupled amplifier whose output keeps failing to one rail or the other, whilst in another room a customer on the phone is being told his destroyed speaker is not the amplifier's fault?

Back to the output capacitor then: if its value results in cut at 4 or 5 Hz, its audible effects will have long disappeared by the time the output reaches 40 - 50 Hz, and might even lead to that "grunt" one magazine keeps extoling on behalf of its favoured manufacturer.

The problem we have is in finding a capacitor which treats all frequencies as equals. In the past that would have been easy because the capacitor manufacturer would have been aware that his product was being used as an output capacitor in addition to its other uses. But today such high value capacitors are made to make the manufacturer money in the switched-mode power supplies he hopes his product will be used in.

I will add to the above that I have so far not seen any 100V switched-mode power supplies: this voltage rating being the one used here.

People replacing Quad 303 output capacitors beware!
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 15 Apr 2019 at 10:42am
To a degree the effect of the output capacitor has been eliminated - it doesn't cause the bright harshness - and so I shall retry the snap-in at some later date. I do think however there will be at least a slight advantage using the Philips radials.

Looking into JLH's "Audio Electronics" I noted his use of an output resistor where the inductor usually goes, his reasoning being it negates the non-linearities of the different halves of the output stage, and at least one famous manufacturer has been known to use this.

The value suggested is 0.22 ohms but it needs to have good wattage so I paralleled two 0.33 ohm emitter resistors (all I had that was near), which at 0.165 ohms is reasonably close.

Removing the output inductor however removes protection against speaker cable capacitance (now you'll be able to guess the manufacturer), so I looked into how small the inductor can be made, and came up with 1uH (16 turns of 1mm ECW on an 8 mm former). This is now strung between board output and speaker terminal wiring, and the inductor is damped by a 10 ohm resistor.



While I had the amp apart I decided to include the anti-parasitic resistors: 10 ohms in the driver bases; and 2.2 ohms in the power transistor bases.

Simulation still shows excellent phase (90 degrees plus) and gain (16dB min.) margins, up to 10,000 pF "cable" capacitance with loads from 4 - 32 ohms.

The 100 uF supply decoupling, local to the voltage amplifier, is also now included.

So far bass-a-plenty, and as long as the upper mids/treble doesn't start edging towards that etched harshness, I will be much happier. But as I know all-too-well, the sound can change considerably over the first 100 hours after a modification.

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Post Options Post Options   Thanks (0) Thanks(0)   Quote DogBox Quote  Post ReplyReply Direct Link To This Post Posted: 16 Apr 2019 at 3:31am
Very Nice! I do hope you are still considering doing this as a Kit..? 
Apparently Neville Theil did something similar on the output coming up with 6.8uH in parallel with 6.8ohm [1W] and a 150nF 250VAC cap tying it to ground was able to overcome the cable capacitance. Still trying to read the right things in my quagmire of literature... 
Think i'll go and put a record on to clear the head! 
Any chance of a Phono Low-Output-MC Amp Kit on the lines of the Genera? Maybe Genera M.C. [for Master Ceremony!] 

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Post Options Post Options   Thanks (0) Thanks(0)   Quote BackinBlack Quote  Post ReplyReply Direct Link To This Post Posted: 16 Apr 2019 at 8:39am
Just listen, if it sounds good to you, enjoy it.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 16 Apr 2019 at 12:17pm
Originally posted by DogBox DogBox wrote:

Apparently Neville Theil did something similar on the output coming up with 6.8uH in parallel with 6.8ohm [1W] and a 150nF 250VAC cap tying it to ground was able to overcome the cable capacitance.


What was he trying to do? Make an oscillator?

I think in this topic added to my years of doing amplifier design (and developing other designs) I must have covered every possible configuration.

All I am able to conclude is that people like it BRIGHT.

Another thing that will be noticed when simulating amplifiers into real loads is the 1kHz (or thereabouts) upward hump and that's going to sound BRIGHT.

Actually there is absolutely nothing that can be done about it, except forcing the amplifier to behave by using a very stiff power supply (as per the Proprius) or by using tone controls!

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Post Options Post Options   Thanks (0) Thanks(0)   Quote BackinBlack Quote  Post ReplyReply Direct Link To This Post Posted: 16 Apr 2019 at 12:23pm
Perhaps there's a lot of dull speakers and cartridges out there.
Just listen, if it sounds good to you, enjoy it.
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