Shindo MAZERIS BELLEVUS MODEL 454-2xAttenuator-How works this Circuit? 6067+12BH7

[tiefbassuebertr]

Such a circuit for a line pre-amplifier I have never seen before.
There are two negative feedback loops from the output:

1) One of the input of the last stage (470 K)
2) One to the cathode of the 6067 - first stage (100K+220K parallel in series with 2uF capacitor)

It looks a bit like a NDFL

The following questions arise for me to the attached schematic diagram (drawing and measure errors are not to rule out):

1) What is the aim of two volume controls (one on front site and one at the rear panel) ?

2) What is the name of the circuit topology behind the second volume control (last two stages with 12BH7 - at fist look a SRPP, but it isn't)?

3) The second volume control is inside of the globale NFB loop. Which consequenses are to expect in case of the different attenuator positions there?

4) The couple capacitor in front of the second volume control is a Sprague ceramic disc with a diameter of approximately 1 inch. What could be the advantage?

about
http://www.hifido.co.jp/KW/G0103/J/0...5468-61439-00/
you will find some pics about this preamp.

[kevinkr]

There aren't actually two negative feedback loops in this design, the 470K resistor is part of the bias network for the 12BH7A, it is arguably partially bootstrapped by this connection..

The output stage is a white cathode follower..

The volume pot on the rear varies the available open loop gain that can be applied to the feedback loop, allowing one to vary the feedback margin. Presumably this would allow one to tailor the harmonic content of the output signal. Note that as designed it is quite possible to set it in such a fashion that either massive distortion or no sound at all will result. (As you approach the low end of the pot setting.)

The ceramic coupling cap is a fashion statement, and a highly non-linear one at that.

This seems like a fairly questionable design, the best I can say is that it is clearly intended to "interpret" the program material rather than reproduce it accurately...

Originally Posted by tiefbassuebertr

Such a circuit for a line pre-amplifier I have never seen before.
There are two negative feedback loops from the output:

1) One of the input of the last stage (470 K)
2) One to the cathode of the 6067 - first stage (100K+220K parallel in series with 2uF capacitor)

It looks a bit like a NDFL

The following questions arise for me to the attached schematic diagram (drawing and measure errors are not to rule out):

1) What is the aim of two volume controls (one on front site and one at the rear panel) ?

2) What is the name of the circuit topology behind the second volume control (last two stages with 12BH7 - at fist look a SRPP, but it isn't)?

3) The second volume control is inside of the globale NFB loop. Which consequenses are to expect in case of the different attenuator positions there?kist

4) The couple capacitor in front of the second volume control is a Sprague ceramic disc with a diameter of approximately 1 inch. What could be the advantage?

about
???????? McIntosh/JBL/audio-technica/Jeff Rowland/Accuphase/?/?¾?????
you will find some pics about this preamp.

[DF96]

A strange circuit. It has the standard mistake which everyone seems to make: volume pot slider goes straight to grid, so pot gets noisy and valve has no bias when pot fails. Putting a volume pot inside a feedback loop means that it is an effects box, not a serious preamp.

The RIAA preamp section is running the ECC83 rather too near the region of grid current at below 1V bias, so distortion is quite likely at the HF end where the cartridge impedance rises. The 3K cathode resistor will inject thermal noise. The 22K anode resistor seems rather too small for an ECC83. Is that really a 1G resistor in the RIAA feedback network?

I enjoy pulling a commercial circuit to pieces. Does that mean I am a nasty person? If you were teaching valve audio you could set this as an exercise for students: one mark for every circuit mistake they find.

[kevinkr]

Originally Posted by DF96

<snip>

I enjoy pulling a commercial circuit to pieces. Does that mean I am a nasty person? If you were teaching valve audio you could set this as an exercise for students: one mark for every circuit mistake they find.

This one provides so much fodder, an abject lesson in questionable audio design. We all make mistakes, just usually not so many in one place.. (I have more than a few I'd like to keep buried.. ) Nasty? Nah, maybe more of a hard bitten realist.. IMHO..

Or barring shear incompetence maybe it's simply that one designer's mistake is another's deliberate choice..

[tiefbassuebertr]

Thank you very much for this explantations - at my own view is this the main reason, why I don't get schematic diagrams from the most manufacturers.
I will remove the obviously design errors.
The 1 G-Ohm resistor in the RIAA network isn't a misprint; the colors are brown-black-blue and the measured value is also 1 G-Ohm (exactly 1,06 G-Ohm).

[DF96]

We all make mistakes. When I worked as a computer programmer I insisted that everyone's work was inspected by two or three of their peers in a code review. As well as picking up mistakes it also helped us learn from each other. Maybe the audio industry should do this, although having incompetent people inspect each other's work does not achieve very much.

PS a 1G resistor could mean that the RIAA response will depend on the weather, as atmospheric moisture is bound to affect its value unless it is sealed in a box (but even then there will be leakage paths around the outside of the box). A truly daft design!

[tiefbassuebertr]

Originally Posted by DF96

We all make mistakes. When I worked as a computer programmer I insisted that everyone's work was inspected by two or three of their peers in a code review. As well as picking up mistakes it also helped us learn from each other. Maybe the audio industry should do this, although having incompetent people inspect each other's work does not achieve very much.

PS a 1G resistor could mean that the RIAA response will depend on the weather, as atmospheric moisture is bound to affect its value unless it is sealed in a box (but even then there will be leakage paths around the outside of the box). A truly daft design!

By my version of phono preamp I will realize an opposite approach: very low impedance RIAA network for caps in the aera between 10uF until 20uF for lowest possible microfonic effects.
Unfortunately this RIAA head amp will looks like as a big power amp device therefore. Cause many effects I will respect the performing will take several time mainly through the analyzing of the pro and cons of the various RIAA equalizing topologies. Do you know, what are the best threads about this topic here on diyaudio?

[kevinkr]

Originally Posted by tiefbassuebertr

Thank you very much for this explantations - at my own view is this the main reason, why I don't get schematic diagrams from the most manufacturers.
I will remove the obviously design errors.
The 1 G-Ohm resistor in the RIAA network isn't a misprint; the colors are brown-black-blue and the measured value is also 1 G-Ohm (exactly 1,06 G-Ohm).

Brown-Black-Blue is actually 10M not 1G, anything over 30M is going to be fairly uncommon. 1G resistors are extremely rare in the field although they do exist..

[tiefbassuebertr]

Originally Posted by kevinkr

Brown-Black-Blue is actually 10M not 1G, anything over 30M is going to be fairly uncommon. 1G resistors are extremely rare in the field although they do exist..

Yes, thanks for that advice. In the attachement you will find all the corrections, that I have found in the schematic by troubleshooting on the device. I have marked the changes with a black triangle.
Who knows the values of the Mallory multi-capacitor for filtering the heater voltage?

[DF96]

Even at 10M that feedback resistor could still be wrong. It ought to be something in the region of 1.5M, to give the 20dB RIAA LF shelf with the 150K. With infinite open-loop gain it would be 1.377M, but two '83 triodes don't quite give infinite gain - maybe a few hundred or even a thousand - so the resistor has to be bigger. If 10M is indeed the correct value, then the RIAA LF response will change as the valves age as there is virtually no feedback at the LF end. If 10M is too big, as I suspect, then this preamp might be fine for 'one-note-bass' when listening to reggae as the response will carry on rising past 50Hz (going downwards) instead of levelling off. You would also need a perfect turntable with perfect warp-free records. My guess is that the 'designer' is one of those who 'designs' by ear instead of using a calculator.