making of a triode coupled RIAA preamp
 

Good evening Forum,
maybe you noticed this thread:
https://www.vintage-radio.net/forum/...ad.php?t=16509
You can find fundamentals there how a dc coupled valve amp can be realized.
The fundamental component is a triode.;)
The triode makes it possible to replace all kind of coupling capacitors, coils, transformers, and resistors. I will call this triode "coupling triode".
As you know from the link, the input voltage at the plate is divided micro
times by the coupling triode if its grid is decoupled to ground.
Decoupling the grid of the triode to its cathode, the signal is divided by one
because the triode works in a "diode mode".
To get a "RIAA coupling" between the stages, the grid of the coupling triode
is coupled to ground or to cathode depending in the frequency.8-)
The coupling triode makes a RIAA coupling now. I will call this triode "RIAA triode".
In the attachements you can see the fundamental schematic and the signal
flow schematic. Please note that there are only valves (triodes) in the signal
path, no caps, transformers, coils or resistors etc. This is unique, isn't it???.

I want to use the well known valves ECC83 and ECC82.
The power supply is solid state and makes 12V6 for the heaters,
+250V and -50V for the valve stages, -12V for the biasing.
the attached pics show the making of the power supply.
If it is of interest, I can explain it too.:)

Kind regards,
Darius

Re: making of a triode coupled RIAA preamp
 

Hi, I made a parts description for you. Hope it helps understanding what is going on. The complete schematic followes soon.

Parts description for the dc coupled triode RIAA preamp 2007

R1 pick up load resistor
R2 RFI Filter resistor (optional)
R3 / C2 positive supply voltage smoothing
R4 kathode resistor cathode coupled amp V1 V2
R5 / C3 negative supply voltage smoothing
R6 / R10 kathode resistor V4 C8 bootstrap capacitor
R7 / C4 positive supply voltage smoothing
R8 bias resistor V2 V3
R9 / C5 offset voltage smoothing first stage
R10 /R6 kathode resistor C8 bootstrap
R11 coupling valve V4 bias voltage feed resistor
R12 / C6 RIAA timing components for RIAA Valve V4
R13 / C9 positive supply voltage smoothing
R14 / C12 positive supply voltage smoothing
R15 bias resistor V6 V7
R16 kathode resistor cathode coupled amp V5 V6
R17 / C10 negative supply voltage smoothing
R18 / R20 kathode resistor V8 C15 bootstrap capacitor
R19 / C11 offset voltage smoothing second stage
R20 / R18 kathode resistor V8 C15 bootstrap capacitor
R21 / C14 RIAA timing components for RIAA Valve V8
R22 coupling valve V8 bias voltage feed resistor
R23 / C16 positive supply voltage smoothing
R24 cathode resistor for current source V9
R25 / C17 negative supply voltage smoothing
R26 / R27 grid bias divider current source V9
R27 / R26 grid bias divider current source V9
R28 output series resistor (optional)

C1 pic up load cap (optional)
C2 / R3 positive supply voltage smoothing
C3 / R5 negative supply voltage smoothing
C4 / R7 positive supply voltage smoothing
C5 / R9 offset voltage smoothing first stage
C6 / R12 RIAA timing components for RIAA Valve V4
C7 bias voltage decoupling to kathode of V4
C8 bootstrap capacitor for coupling valve V4
C9 / R13 positive supply voltage smoothing
C10 / R17 negative supply voltage smoothing
C11 / R19 offset voltage smoothing second stage
C12 / R14 positive supply voltage smoothing
C13 bias voltage decoupling to kathode of V8
C14 / R21 RIAA timing components for RIAA Valve V8
C15 bootstrap capacitor for coupling valve V8
C16 / R23 positive supply voltage smoothing
C17 / R25 negative supply voltage smoothing
C18 grid bias voltage smoothing V9

V1 cathode follower, V1 V2 cathode coupled amp
V2 cathode input amp, V1 V2 offset voltage difference amp
V3 high impedance load for V2, kathode follower, plate voltage source for V4
V4 RIAA coupling triode
V5 cathode follower, V5 V6 cathode coupled amp
V6 cathode input amp, V5 V6 offset voltage difference amp
V7 high impedance load for V6, kathode follower, plate voltage source for V4
V8 RIAA coupling triode
V9 high impedance current source for V10
V10 low output impedance cathode follower, output stage

Thanks for looking,
kind regards,
Darius

Re: making of a triode coupled RIAA preamp
 

VERY interesting work Darius! I am still trying to follow it.
You seem to have a cathode follower input, DC coupled to an SRPP stage. That is then DC coupled (through the triode) to another cathode follower and SRPP. That is then DC coupled through a triode to the final stage- another cathode follower but with triode constant current sink.

I will print your schemtic and BOM out, and look more carfully!

Re: making of a triode coupled RIAA preamp
 

Hmm, you say that in your "triode transformer" the AC component is divided by mu. Surely that causes very heavy loss in gain?
Surely the same could be acheived with a resistive divider, DC coupled to a cathode follower to get a low Zout, but would cause less attenuation of the AC signal?

Also, I would argue that there are resistors and capacitors in the signal path, because all of the components play some part in signal flow. The signal does not simply follow the path you have drawn, like a mouse in a maze.
However, I love your design, and will seriously look into building it (I need a valve phono stage!)

Re: making of a triode coupled RIAA preamp
 

Good evening daviddeakin,
yes, that is right. :)

Kind regards,
Darius

Re: making of a triode coupled RIAA preamp
 

Ok, I have looked more closely at the schematic now, and I have a few questions!
What is the purpose of V1? Why not input the ignal directly to the grid of the SRPP (V2). The same is tru of V5.

What it the purpose of R9? At first I thought it was for feedback, but the grid of V2 is already fully decoupled to ground via C5. The same is true of R19/C11.

What is the purpose of R27? Surely the grid of V9 should be fully decoupled via a large capacitor, or unwanted feedback will occur?

Re: making of a triode coupled RIAA preamp
 

Hi, a lot of questions.
I just finished the schematic and the parts value list.
I am sure this will answer some questions.
Please note that triode V9 1/2 ECC83 has a positive grid.
Have a look at the voltage droppings at this stage.
This increases the output resistance of a triode.
Very importand:
The DC current of V1/V5 is ten times of the DC current of V2/V6
and the DC current of V3/V7 is a bit more than two times of the V2/V6 current.

daviddeakin wrote:
"Also, I would argue that there are resistors and capacitors in the signal path, because all of the components play some part in signal flow. The signal does not simply follow the path you have drawn, like a mouse in a maze."

I can not see another way than the "red line" for the signal flow???.
Please note that supply rails are signal ground;).

To minimize the influence of surrounding parts in the signal you must follow some triode design rules:

Kathode follower: The pull dowen resistor in parallel to the output load must be much (say 10 times) higher than the mutal conductance.

Kathode base/ grid base: The pull up resistor in parallel to the output load must be that high that the triode reaches micro gain.

This is why I made these current settings in the cathode coupled stages8-).



Component values for the dc coupled triode RIAA preamp 2007
R1 47K
R2 470R (optional)
R3 22K
R4 22K
R5 1K
R6 47K
R7 10K
R8 47K
R9 1Meg
R10 47K
R11 2M2
R12 22K
R13 22K
R14 10K
R15 47K
R16 22K
R17 1K
R18 47K
R19 1Meg
R20 47K
R21 22K
R22 2M2
R23 10K
R24 2K2
R25 470R
R26 2K7
R27 11K two 22K resistors in parallel
R28 2K2 (optional)
RV1 22K
RV2 22K
C1 220p (optional)
C2 4µ7
C3 47µ 63VDC
C4 4µ7
C5 1µF MK 50VDC
C6 18n
C7 1µF MK 50VDC
C8 1µF MK 50VDC
C9 4µ7
C10 47µF 63VDC
C11 1µF MK 50VDC
C12 4µ7
C13 1µF MK 50VDC
C14 220pF
C15 1µF MK 50VDC
C16 4µ7
C17 47µF 63VDC
C18 100µF 16VDC
V1 ½ ECC83
V2 ½ ECC83
V3 ½ ECC82
V4 ½ ECC82
V5 ½ ECC83
V6 ½ ECC83
V7 ½ ECC82
V8 ½ ECC82
V9 ½ ECC83
V10 ½ ECC83

Kind regards,
Darius

Re: making of a triode coupled RIAA preamp
 

Never mind. I was really arguing that because all current, (AC or DC) is suppled by the power supply, there is no such thing as a "siganl path", since ALL components are involved with the flow of current. It's not important.

I have been looking more closely at the theory. You say the AC signal through the tride transformer is divided my mu. This is not so; the AC signal gain through it can be given by:
Av= (ra + Ra) / ra

where:
ra = triode internal impedance
Ra = load resistance (R10 / R20)

To minimise signal loss, it is therefore desirable to make ra large, and Ra small. One method of making ra large would be to use a pentode, but that adds complexity. Another method would be to lower the heater voltage. This will considerably increase ra for a given bias voltage, but also increase linearity!

I still do not see a reason for V1, when you could input the signal directly to the grid of V2, thereby eliminating distortion caused by V1?

Re: making of a triode coupled RIAA preamp
 

Hi daviddeakin, you wrote
"Never mind. I was really arguing that because all current, (AC or DC) is suppled by the power supply, there is no such thing as a "siganl path", since ALL components are involved with the flow of current. It's not important."

Sorry, for my this is very importand.
I am a Radio and TV serviceman, finding and following the signal path is essential to do this work. Instruments like the "signal follower" for example are made to find where the signal path is interrupted.
The aim was to develop a circuit that makes it possible to have only valves say triodes in the signal path. And this is done here.8-)

In
https://www.vintage-radio.net/forum/...ad.php?t=16509
the theory of triode coupling is explained. I verified the results with practical tests!!! Please move your questions in triode coupling fundamentals to this https://www.vintage-radio.net/forum/...ad.php?t=16509 thread.
Did you read about the "diode mode" of the coupling triode in the starting post here?;)

In:
http://www.tubecad.com/2007/05/blog0107.htm
http://www.tubecad.com/2007/05/blog0108.htm
You can find some cathode coupled amp fundamentals.
My cathode coupled amp does not work in the described mode there are some aspects to add.

In:
http://www.tubecad.com/articles_2002...t_2/index.html
you can see three passive equalization methodes.
I am using the second here. (30dB finst RIAA 30dB second RIAA and Buffer)

Kind regards,
Darius

BTW isn't the thing with the heater cathode voltage vice versa???.

Re: making of a triode coupled RIAA preamp
 

I found a good RIAA website:
http://www.platenspeler.com/backgrou...kground_1.html

Please note that the RIAA coupling triode provides you an improoved and enhanced RIAA.;)

Kind regards,
Darius

Re: making of a triode coupled RIAA preamp
 

Hello,
for better understanding how the RIAA coupling triode acts in the circuit I made some drawings for you.
In the coupling triode AC modes you can see first the "triode transformer mode" explained here:
https://www.vintage-radio.net/forum/...ad.php?t=16509
In this mode the input voltage (and resistance) is divieded micro times.

In the "diode mode" (see post#1) input and output is the same.

The "vari u" mode makes it possible to get a reduced micro to get less damping in triode coupled stages for example.

The RIAA-triode operates in the "filter mode". In my case the RIAA characteristec is made in two steps.

In the next drawing you can see the block diagram.
The flat part of the RIAA curve and the enhanced RIAA (eRIAA) is
set by micro of the coupling triode. Note that triodes micro has no aging.

Kind regards,
Darius

Re: making of a triode coupled RIAA preamp
 

Hi, here are some updates and pics for you.
Thanks for looking.:)

Kind regards,
Darius

Re: making of a triode coupled RIAA preamp
 

Nice job Darius!

I'm very taken by the "brick" style of modular construction.

If you had enough space you could build a complete system on the same principle.

Regards,

Re: making of a triode coupled RIAA preamp
 

Hi Brian,
when the RIAA is ready build, I will make a new valve amp for it. Maybe a classic Loftin White
or a new designed DC coupled OTL. Using the coupling Triodes in the "transformer mode" and in the "vari u mode". This is an interestig thing (for me) because there will be no Caps, Coils and Transfromers in the signal path from the pic up to the speaker.:)

In the attachements you can see how the chassis is made and the "bricks"
are mounted on the chassis.

Kind regards,
Darius

Re: making of a triode coupled RIAA preamp
 

I think we misunderstand one another. In the case of a preamp, we are usually only interested in voltage amplification; the flow of current between stages is usually zero, or of no interest.
Therefore, although you can "trace a signal" through a circuit, all you are really doing is viewing the voltage changes at different points in the circuit- it is a modulated power supply, and nothing more. ALL capacitors in the circuit contribute to the voltages changes at each stage, and therefore ALL capacitors have an audible effect to some degree, even though you "red line" doesn't pass through any.

Re: making of a triode coupled RIAA preamp
 

Not convinced. The current flow is primarily controlled by electrostatics and the mechanical construction, whereas the heater controls the size of the pool of available electrons around the cathode.


But agree regardings signal path - every component between the "signal path" and signal ground will have an influence.


TTFN,
Jon

Re: making of a triode coupled RIAA preamp
 

It is indeed a little known side effect of reducing the heater voltage that it also increases the linearity of the valve, at the expense of gm. ra of course increases.
(I actually had my Ra and ra theory backwards in my earlier posts, so ignore that, I was being stupid). Obviously once you drop below about 75% of the normal heater voltage all bets are off, but for small reductions it holds true and is well worth doing in hifi because the reduction in gm is negligeable, while the additional increase in lifespan is also useful.

Re: making of a triode coupled RIAA preamp
 

Hi,
everything has an "audible effect" to "some degree". This is why some people buy gold plated mains plugs etc.
But sorry this is not the theme I want to discuss.
For me it is a great progress if coupling caps and transformers can be replaced by a triode.
A triode amp is what I want.
A good circuit design can minimize the "audible effect" of components sorrunding the signal path.

Kind regards,
Darius

Re: making of a triode coupled RIAA preamp
 

Hi, the RIAA preamp is working fine.
I am making a website for it http://Radio-Darius.blogspot.com
It will be translated to english when it is finished.
Thanks for looking.

Kind regards,
Darius