Mr. Schmitz
I think you have a lot of confusion in your mind.
And a superficial approach to the problems of a phono stage.
In addition the lab test doesn't give you the quality of the sound but the quality of the project following the riaa indication.
In addition one of the most important aspect is the s/n ; with a good pcb you can get good value for everyone want to build a phono stage.
For me, just to be clear, I have a lot of people that listen my project before the final versione and before the publication on Audioreview magazine.
Ciao
Walter
I think you have a lot of confusion in your mind.
And a superficial approach to the problems of a phono stage.
In addition the lab test doesn't give you the quality of the sound but the quality of the project following the riaa indication.
In addition one of the most important aspect is the s/n ; with a good pcb you can get good value for everyone want to build a phono stage.
For me, just to be clear, I have a lot of people that listen my project before the final versione and before the publication on Audioreview magazine.
Ciao
Walter
This is my variant of a moving-magnet phono amplifier with valves. Due to Miller effect in the first stage, it is not suitable for cartridges that require a very low load capacitance: you get about 90 pF with all switches off. The 47 kohm input resistance is made with a feedback technique to reduce the equivalent input noise current. I built it with point-to-point wiring, simply because I didn't want to design a PCB. The neon lights are used as overvoltage protection, they work best when there is some light shining on them. It is my first and so far only electronic circuit with no transistors or semiconductor diodes.
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Its much better to put the RIAA in front of the two first amp stages. I do understand the intention, but its simply wrong. Use a strong second stage tube with some current (12AU7) at second stage and the network will not sound cloudy. Job done.
Furthermore, an ECC83 isn't the best choice for a cathode follower. This stage should deliver current as well, no high mu valves.
Split load in the first EF86? Makes no sense to me. Nobody wants to reduce gain in that stage by split load. A strange concept, never seen before in any MM phono amp. You should switch ECC83 for lower gain tubes. Thats the way to do it and makes needless the split load.
The first EF86 needs 47uF buffer? Another strange thing, old designs used 1-2uF here. More isn't always better. In this case, more is more worse.
The whole concept uses plenty of passive components with no visible success.
I could offer the absolute same (or better) performance with 1/3 of passive components.
Looks like a pre designed from someone who has never studied the famous and best designs of the past. This is bad design practice with some real strange design faults added.
I'll give a grade 4 minus because it may work but its so poorly designed that tears come to any good and efficient audio designer.
In University the course of tube design wouldn't have been passed successfull if thats the master work.
Furthermore, an ECC83 isn't the best choice for a cathode follower. This stage should deliver current as well, no high mu valves.
Split load in the first EF86? Makes no sense to me. Nobody wants to reduce gain in that stage by split load. A strange concept, never seen before in any MM phono amp. You should switch ECC83 for lower gain tubes. Thats the way to do it and makes needless the split load.
The first EF86 needs 47uF buffer? Another strange thing, old designs used 1-2uF here. More isn't always better. In this case, more is more worse.
The whole concept uses plenty of passive components with no visible success.
I could offer the absolute same (or better) performance with 1/3 of passive components.
Looks like a pre designed from someone who has never studied the famous and best designs of the past. This is bad design practice with some real strange design faults added.
I'll give a grade 4 minus because it may work but its so poorly designed that tears come to any good and efficient audio designer.
In University the course of tube design wouldn't have been passed successfull if thats the master work.
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Absolute nonsense. Almost every passively equalized phono preamp will place at least part, if not all, of the RIAA filter between two gain stages. If equalization is split, with the 75 microsecond time constant in a separate filter, that will often appear after the second gain stage, with perhaps a cathode follower buffer after that.
I don't know how you can make such categorical statements as these with no justification at all. I'd be curious to see even one reference from a reputable engineer to support what you are saying.
By all means, please show us your design. I'd really like to see it.
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Both the active and passive EQ methods have ("shock") both pluses and minuses. 
A big plus that favors passive EQ is greater overload resistance.
As is always the case when multiple, reasonable, methods are being cross compared, implementation details will be critical. For instance, a well executed "full wave" voltage doubler PSU will be better than a badly executed true full wave setup. Vice versa is equally true.
A big plus that favors passive EQ is greater overload resistance.As is always the case when multiple, reasonable, methods are being cross compared, implementation details will be critical. For instance, a well executed "full wave" voltage doubler PSU will be better than a badly executed true full wave setup. Vice versa is equally true.
I built this with components i had :
An externally hosted image should be here but it was not working when we last tested it.
And have the RIAA's highly variable impedance load the cartridge?
And attenuate a weak signal before amplification? That puts lots of Johnson noise from the RIAA network in series with the signal.
No, not a good idea, would be a poor performer. Or did you mean use an RIAA feedback network on the first stage?
That does look pretty good although I'm sure Schmitz77 will soon tell you exactly what's wrong with it....
That would be a misunderstanding, of course.
The RIAA between the first two stages.
The attenuator in a full function tube pre has to be in the middle, at the output of low level section.
RIAA on first stage only has been done by Mullard.
Where can I study this particular Mullard design? Is the circuit diagram available somewhere on this Forum?
This is probably what is being referred to:
Mullard Stereophonic Pre-amplifier
For the record, this circuit uses active (feedback) equalization, which was popular in the 1950s and 1960s (when this design was published) but has largely been replaced in modern tube preamps with passive equalization. Like Eli mentioned earlier, there are pros and cons to both approaches but that discussion is probably beyond the scope of this thread.
This circuit is interesting from a historical perspective but would not be considered typical by modern design standards. The suggestions presented earlier in this thread would be more representative of contemporary designs and are all worthy of consideration if you are looking to build a preamp for yourself.
Mullard Stereophonic Pre-amplifier
For the record, this circuit uses active (feedback) equalization, which was popular in the 1950s and 1960s (when this design was published) but has largely been replaced in modern tube preamps with passive equalization. Like Eli mentioned earlier, there are pros and cons to both approaches but that discussion is probably beyond the scope of this thread.
This circuit is interesting from a historical perspective but would not be considered typical by modern design standards. The suggestions presented earlier in this thread would be more representative of contemporary designs and are all worthy of consideration if you are looking to build a preamp for yourself.
I think it is largely a matter of taste. Some people like the SRPP signature while others do not. The majority avoids listening anyhow
The SRPP has some remarkable qualities: it is really cheap to build, provides a low output impedance and is really difficult to screw up. Hey, even AN use it
None of which of course matters if you hate the way it sounds.
This certainly looks original. Could you say a few words about your design decisions? Specifically about the input stage.
Actually it's Broskies Aikido circuit without the "Aikido" noise cancellation, not an SRPP.
This is more than a few words... If you want even more information, please look in Linear Audio volume 4.
I wanted to build a circuit with no semiconductor diodes or transistors at all (just for the hell of it). As I also didn't fancy using batteries or mercury vapour rectifiers, that meant using an AC heater supply. To keep hum under control, I chose a valve that's specifically designed for low-level audio stages and that has an exceptionally small heater to control grid capacitance, the EF86, as input valve.
For a discrete phono preamplifier, I would normally use the Hoeffelman and Meys configuration, which is a feedback structure with series and shunt feedback that can provide the RIAA correction and realize the 47 kohm input resistance with less noise than a 47 kohm termination resistor. (See Jean M. Hoeffelman and Rene P. Meys, "Improvement of the noise characteristics of amplifiers for magnetic transducers", Journal of the Audio Engineering Society, vol. 26, number 12, December 1978, pp. 935...939 and Ernst H. Nordholt, "Comments on "Improvement of the noise characteristics of amplifiers for magnetic transducers"", Journal of the Audio Engineering Society, vol. 27, number 9, September 1979, p. 680.) However, for minimum hum, the cathode of the EF86 has to be AC grounded, as this eliminates the effect of heater-to-cathode capacitance and heater emission. Series feedback is difficult when the cathode must be AC grounded, so I had to come up with something else.
In the end I decided to use the input valve as an open-loop inverting amplifier with a flat response over the audio band. A shunt feedback around this inverting stage could then set the input resistance with less current noise than a plain old termination resistor. For the RIAA correction, I connected the output of the inverting amplifier to an inverting feedback amplifier with RIAA response.
As this second amplifier only needs to provide a part of the gain of the phono amplifier, it gets easier to make enough loop gain compared to a normal actively equalized phono amplifier. Besides, inverting amplifiers don't have the infamous +1 term that causes an extra zero somewhere in the ultrasonic range (not that I consider that to be a big deal, but it is nice to get rid of it).
Getting back to the first stage, I wanted to connect the EF86 as a triode to get rid of its partition noise and I wanted the gain to depend on its mu rather than its gm. The mu depends far less on the bias current than the gm and it doesn't depend that much on the cathode emission, but more on the geometry of the grids. That is, I expected the mu to be more linear and reproducible and less sensitive to aging than the gm.
In order to make the gain depend on mu, the triode-connected EF86 has to see a load that has a high impedance compared to its internal resistance (internal resistance when triode-connected, that is). A voltage follower (ECC83 cathode follower) between the EF86 and the inverting feedback amplifier with RIAA correction ensures that the input resistance of the inverting feedback amplifier doesn't load the first EF86. A big capacitor from the voltage follower to the midpoint between the anode resistors of the triode-connected EF86 bootstraps those resistors out of the equation. The AC coupling caps between the EF86 and the ECC83 cathode follower and at the input of the inverting feedback amplifier are arranged such that they together form a second-order Butterworth high-pass at about 6 Hz.
The bias current of the triode-connected EF86 was set to what I measured to give the lowest RIAA- and A-weighted noise. Higher currents lead to more 1/f noise and lower currents to more white noise, so there is an optimum that depends on the weighting. By the way, the noise performance of a triode-connected EF86 is good enough for moving magnet, but it is not spectacular. When you only care about noise and not about hum, an ECC83 or an E88CC can easily outperform it.
The active part of the inverting RIAA-corrected feedback amplifier uses a similar construction as the open-loop inverting amplifier, but with the EF86 used as a penthode to get maximum loop gain.
I don't remember why I used an ECC83 for V2, because of its high mu, its high cathode-filament voltage rating or because I happened to have some lying around?
Except for the input noise current, I don't know if this circuit outperforms more conventional circuits. I know it is good enough for me and that it was fun to design and build.
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