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Topic: Another JFET phono preamplifier (Read 3329 times) previous topic - next topic
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Another JFET phono preamplifier

Hello guys! I remembered recently about a Romanian magazine since 1994 where was presented an interesting phono preamplifier based on FET transistors. As previously most listeners preffered to buy or construct either "Le Pacific" or "Boozhound" preamplifier, I decided to come in help for them with this diagram.

Short description:
"The preamplifier for the magnetic cartridge presented can satisfy a series of lately wishes such as:
-very good signal/noise raport thanks to using the FETs;
-very low THD for a high gain and a high output voltage;
-high capacitance for preloading the signal peaks.

Technical characteristics:
-Gain at 1KHz = 40dB;
-V out = 250mV;
-S/N = 82dB;
-overload for the normal output voltage (250mV) = 40dB;
-THD = 0,01%;
-Z in = 220K;
-minimum Z out = 1K.

The preamplifier is built after the pattern of an OA equipped with FETs at the input in a differential mode. The stage mixes or combines the advantages of the BJT (regard frequency and high amplification) with the low noise of the P-channel FETs.

Regarding to the type of cardridge, C1 can vary between 10-90pF.

Re: Another JFET phono preamplifier

Reply #1 –
What would be the advantage of using discretes vs. just using the OA upon which the circuit is based?

Plus, if it was printed in 1994 then it may be based on a rather old OA design.

Re: Another JFET phono preamplifier

Reply #2 –
Sometimes the discretes components give us for such a correction lower THD.

Re: Another JFET phono preamplifier

Reply #3 –
Seems pretty unlikely though. These days any decent OA should provide THD well below audibility. Plus, IC processing will tend to yield very well matched components, which is hard to achieve with discretes.
 

Re: Another JFET phono preamplifier

Reply #4 –
I agree. THD isn't a valid reason for choosing discretes over integrated OAs. It gets particularly gross when people build simple and old circuits like the Boozhound, which can't compete in terms of THD.

In the case of phono preamps, particularly for moving coil cartridges, a valid reason for discrete parts could be the noise performance. If you know exactly what you're doing, there is still a possibility to improve noise performance beyond even good OAs, by employing discrete components.

Re: Another JFET phono preamplifier

Reply #5 –
A quick check turned up a dual OA with 1.8 nV/rtHz noise for less than $2. I'd like to see anyone top that with discretes.

Re: Another JFET phono preamplifier

Reply #6 –
There are a number of designs with discretes that feature way below 1 nV/rtHz. One of the better examples, admittedly a bit extreme, can be found in the 3rd edition of the Art of Electronics by Horowitz and Hill. They describe a transformerless ribbon microphone preamp that reaches 0.1 nV/rtHz.  Chapter 8.5.9 describes the part selection and design process in quite some detail.

Re: Another JFET phono preamplifier

Reply #7 –
0.1 nV/rtHz is the Johnson noise of a 0.6 ohm resistor. Do you know of any phono cartridges with an impedance this low?

Re: Another JFET phono preamplifier

Reply #8 –
I'm not saying you should use a cheap-terrible preamp, but it seems a little silly to worry about noise, distortion, or frequency response (or RIAA accuracy) when you're playing analog vinyl where the record itself is poor in all of these aspects (when compared to digital).

Of course, excessive hum is a problem that should be dealt with (if you want the best vinyl sound quality), but that's usually related to the power supply or grounding rather than the preamp circuit design.

Re: Another JFET phono preamplifier

Reply #9 –
Quote from: pdq on
0.1 nV/rtHz is the Johnson noise of a 0.6 ohm resistor. Do you know of any phono cartridges with an impedance this low?
No I don't. The amplifier described by Horowitz/Hill is aimed at ribbon microphones, not phono cartridges. The impedance of such a ribbon may very well be below 1 Ohm, which is why ribbon microphones were traditionally equipped with a transformer. It is difficult to come up with a ribbon microphone amplifier design that offers low enough noise without using a transformer, so that's the challenge Horowitz/Hill decided to take.

I agree that MC preamps do not need to go that far, but 1.8 nV/rtHz would be too high for an ambitious design. Hence you'd still consider using discretes, which is the point I was trying to make.

Quote from: DVDdoug on
I'm not saying you should use a cheap-terrible preamp, but it seems a little silly to worry about noise, distortion, or frequency response (or RIAA accuracy) when you're playing analog vinyl where the record itself is poor in all of these aspects (when compared to digital).

You could at least try to do your best to make the preamp disappear from the equation with respect to THD and noise. I agree that there's no point in going over the top with it. The JFET-based preamps shown, however, are quite simplistic. I wouldn't be surprised to find that their deficiencies are audible.

Quote from: DVDdoug on
Of course, excessive hum is a problem that should be dealt with (if you want the best vinyl sound quality), but that's usually related to the power supply or grounding rather than the preamp circuit design.

In a sensitive preamp like this, hum may even be picked up by the PCB wiring. Getting the hum out of the design is probably one of the more interesting challenges for the DIY builder.