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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: 09 Dec 2018 at 10:02am
I expect the PCBs will be with me mid-week, and also a bare metal prototype case!!! So if you find you need relief from the pressures of that great tinsel and holly "holiday", keep calling-in here to witness the birthing pangs of the 1970s design indulgence power amplifier!

Much more interesting than boring old Christmas! And something to enjoy every day of the year!

A super stereo separation dual mono 50WPC stereo amplifier with incredible sound staging, done the way it used to be done.

Differential technology took over and this superb configuration was left to gather dust for nearly half a century. Now it's brought back with up to date transistors and techniques which really showcase how good this configuration really is.

First brought back from the dead in the Proprius, this is the real deal, the complete work with linear (proper) transformer dual power supply.

Too good to miss! Keep reading!
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 10 Dec 2018 at 7:12am
Another Phono Stage Idea

Whilst we wait for the realisation of the power amplifier - there's a lot of boring engineering to do - I thought we'd take another look at what could go into the preamp.

I think for it to be authentic it does need to have a phono equaliser built in. Therefore somebody could use the integrated offering I'm developing as a one box solution.

But you might remember that built-in phono equalisers were always considered to be a compromise? And yes, they probably always will be, not because it can't be made to work - it can - but it would get quite complicated and drive up cost.

The reason being there isn't much room inside an integrated amplifier, or should I say, insufficient room for a through hole component discrete circuit. I am interested in just how far 70s circuitry can be pushed in achieving better sound than there was back then. Could we today take the two transistor stage quite popular back then and make it perform? I truly hope so.

Here's yet another take on the two transistor idea...



And here is its gain and phase plot...



Dashed line = open-loop "envelope"; dashed-dotted line = RIAA replay response taken before output filter; solid line = phase response

Without negative feedback it has gain of about 52dB meaning eventual gain is quite limited. We want at least somewhere in the region of 30 - 40 dB negative feedback to minimise distortion, but that would mean only 12 - 22 dB gain at 20Hz meaning at 1kHz it would be next to nothing. We need gain to dig the signal out of the noise, so we have to balance up noise with distortion, and hope for decent figures on both.

Open loop and because of the "long-tail" effect of the collector voltages on T1 and T2, the collector resistors have a slight chance of pulling up positive excursions of such a small signal with minimal distortion. But it needs to be 3% or lower to obtain something like 1% at 20Hz. Distortion will fall with increasing frequency as the difference between open-loop gain and closed loop gain increases due to the negative feedback containing the RIAA EQ. Most of this distortion will be even second-order so it might not sound bad. And rising distortion in the low bass will give it the "grunt" the reviewers love!

Distortion could fall below 0.1% at 1kHz if we're lucky, and in the upper mids it could fall even further. But that still means we don't have the gain of a modern op-amp phono stage. We must accept we're only going to get around 25 dB (about 17 - 18 V/V) gain. With a typical MM cartridge outputting 1mV per cm per second velocity we can expect around 90mV output. With a low output MM that will be 45mV, so we could make the line stage sensitivity 50mV which would mean attenuating each line input. Often 1970s (and 1960s) amps would do exactly that.

Other interesting comments...

NFB has the effect of bootstrapping T1 so input impedance is generally R1 in parallel with R7 making 48.3k.

R2 and C3 form an RF filter and also the 100pF cartridge load. R2 can be ignored as regards to capacitive loading as it has little effect.

The value of R4 (and therefore R3) is a bit low, and usually around 100k for lowest noise, but that reduces open-loop gain because of the load of T2, so distortion would rise.

R9 will have a job on to drive the NFB EQ at high frequencies on positive excursions, but by making it "long tailed" - more voltage across it than the transistor - gives it a better chance so long as it doesn't have too much signal to swing. Another reason for making it a low gain stage.

C8 is massive because otherwise the stage would feature the crooked bass response often found in earlier generations of this type of stage.

C4 and C7 stabilise the stage to give 90° phase margin and >20dB gain margin.

R12 and C11 continue the RIAA slope which the type of negative feedback cannot. And thinking about that perhaps the impedance seen by R9 won't be too bad at audio frequencies? It is the higher "pop-step" frequencies I am concerned about.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 11 Dec 2018 at 8:45am
One of the chief objections to the two transistor circuit is "its poor driving abilities", and it is often suggested that an emitter follower be used after T2.

But how can its emitter resistor drive anything? It is a resistor and they're passive.

What happens is that the load becomes part of the emitter circuit, it appears in parallel with the emitter resistor. The transistor can keep pulling the load upwards until it runs out of beta, but the resistor and load is only a "return spring".

So the load pulls itself down? Obviously not, and so the negative excursions of the signal receive no benefit. You might then be able to see that using it to bootstrap T2's collector load will only be half effective?

The collector output however, is attenuated by its load because Rc is the output resistance. Actually it's a bit more involved than that because of "Early effect" but boiled down, this here can be imagined as being a high value resistor in parallel with the collector-emitter path, and so if the collector resistor is 4.7k as here, it may reduce it to 4.5k.

The emitter follower which most think is a cure-all, is also a catch-me-out as it can easily oscillate, and resembles a Colpitt's oscillator when the parasitic capacitances are drawn in.

So this is why the two transistor circuit can often surprise. In fact, it did when I first built it as my first magnetic cartridge phono stage in the mid/late 70s.

However, that was using BC109C transistors whose beta approaches 900. There is at least one certain benefit using high beta transistors over a Darlington arrangement, and that is there is only one set of parasitic capacitances to deal with.

The high beta being useful when used for T2 in that it represents a higher impedance to T1 which therefore increases open-loop gain by at least 6dB. This being sufficient to halve distortion.

Unfortunately "the (semiconductor) industry" decided not to bother making anymore high beta transistors, and this usually happens when there's no demand, so what does that tell you about the "hi-fi industry"?

I do have a considerable quantity of BC184C transistors which should accomplish the same, but it is only the B version which has made it to the saviour: the Central Semiconductor production line.

In my opinion it is worth building and that's what I'm going to do.

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Post Options Post Options   Thanks (0) Thanks(0)   Quote morris_minor Quote  Post ReplyReply Direct Link To This Post Posted: 11 Dec 2018 at 12:30pm
Originally posted by Graham Slee Graham Slee wrote:

In my opinion it is worth building and that's what I'm going to do.
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Bob

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Accession MC/Enigma, Accession MM, Reflex M, Elevator EXP, Era Gold V
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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 Dec 2018 at 9:21pm
Tune-A-Sound...

Switching back to the power-amp for a moment and the subject of negative feedback and its effect on the sound, I found another take off point for my additional feedback to the VAS base. By taking a resistor from the output stage mid-point to T2 base I have been able to change the way it sounds to a degree - which is similar to how I tuned the Proprius. I am currently trying the same value as the Proprius uses.

The great thing about using the output stage centre point is that it doesn't ruin the measured distortion spec., at least for 8 ohm use.

However, the change in input sensitivity (being less sensitive) proves this NFB path is working, and is now 810mV where it was around 770mV.

And looking at the cheap 70s amp circuit it can be seen that a similar multi-path arrangement was being used, probably more by accident than design as it included the output voltage trimmer. Maybe I should replace the fixed value with a pot so you can tune-a-sound?

(only joking)
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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 Dec 2018 at 4:38pm
My last post on this topic might have seemed a little flippant, but final tuning is both serious and frustrating to the extreme.

It's the point where the simulator and the test gear (no matter how expensive it was) can no longer help.

And it's all down to the ears and experience which is treated so cheaply these days!. 'Ears and experience' is what gets you great sound, and not the BS which I'm afraid to say the hi-fi press and several manufacturers lowered themselves into multiple years ago. And unfortunately so many believe their garbage that their systems are so far off kilter that adding a product which should sound good, doesn't to them.

So in my 'control room adjusted' auditioning environment I have to use my years of experience to establish a number of solutions for what my ears might not be hearing, or hearing in an off way.

It was in an age where the design under discussion existed in many a "stereo" that I began to realise that the quality of sound affected people's attitudes at venues where music was being played (yes, discos) and a happy crowd was what hostelry owners desired because: 1. it kept the peace; and 2. it made money.

The same applies to listening at home!
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Post Options Post Options   Thanks (0) Thanks(0)   Quote BAK Quote  Post ReplyReply Direct Link To This Post Posted: 14 Dec 2018 at 4:53pm
All very true.
Thank you Graham for staying the course... and pleasing our ears.Big smile
Bruce
AT-14SA, Pickering XV-15, Hana EL, Technics SL-1600MK2, Lautus, Majestic DAC, Technics SH-8055 spectrum analyzer, Eminence Beta8A custom cabs; Proprius & Reflex M or C, Enjoy Life your way!
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