Solid state phono preamp design philosophy

[Rembulan]

Heloo,
I dont see any schematic ? nobody brave enough to stand in front of the classs to write down the schematic.
I am afraid this thread become another what pavel called xxxxxxPUB

[mlloyd1]

original poster started a "discussion"
eventually, schematics may develop.
be patient ....
or propose something yourself.

back on topic:

as for me, i'm just now unpacking my phono equipment after moving a few years ago. also, no moving coil, so maybe i shouldn't comment at all.


i haven't done any discrete designs (yet) that i consider complete enough to evaluate; only op amp designs at this point. i played with active eq, passive eq, and combination. i ended up settling on a passive eq system with two gain blocks: front end AD745, passive eq, then OP637. rails provided by zener buffered by emitter follower.
sealed in an aluminum box. transformer, rectifiers, initial filter caps in remote box (not with preamp circuitry).

although i am ready to play around with some discrete designs (you guys did some great work in the blowtorch thread!), i have to say the design i saw with the AD745 feeding the AD844 with riaa eq at pin 5 of AD844 was quite intriguing. i forgot where i saw it - it's a commercial kit i think. I like the AD844 but haven't done anything at all with it yet.

mlloyd1

Originally posted by Rembulan
Heloo,
I dont see any schematic ? nobody brave enough to stand in front of the classs to write down the schematic.
I am afraid this thread become another what pavel called xxxxxxPUB

[PMA]

Yes, I did not intend to attach a schematic, just to start open discussion on topic.

Regarding OPA637, it is an excellent opamp, but remember that in case of active equalization we need unity gain stable device, unless we introduce additional resistor, that influences HF rolloff. Then further LP pole must be added to get appropriate frequency response.

[maney]

Originally posted by PMA
... we need unity gain stable device, unless we introduce additional resistor, that influences HF rolloff. Then further LP pole must be added to get appropriate frequency response.

Unless you like the idea of adding the "missing" breakpoint to correspond to the fact that the mastering lathes do not apply an infinite amount of HF boost. IIRC, the consensus is that 50KHz is a good compromise of the admittedly somewhat varied practice.

-- Martin

[Onvinyl]

Hi,
This additional pole-compensation (50kHz) is overestimated. More often than not, I liked it better when it wasn't present. Or, still more often, I did not hear a difference.
Rüdiger

[jcx]

a high frequency zero is advocated by many people:

http://www.hagtech.com/pdf/riaa.pdf

there have been lots of threads on riaa preamps - do we need to repeat them all or could people point out a few of the better ones to advance to base for this thread?

less than a year old:

riaa circuit
(I look esp. for Nelson Pass' remarks)

[Onvinyl]

Hi,
currently, I'm trying variations of the shown input stage:

the nmat and pmat transistors are actually a that340-transistor array, R1 is currently 10k. This is followed by an passive riaa network. I'm experimenting with mosfets in place of Q2/Q6 and with a DC servo.

The second stage is a blowtorchstyle gain stage but not balanced. The sound ist very detailed and punchy. It is not balanced, but really worth exploring.

Rüdiger

[andy_c]

Originally posted by PMA
Regarding OPA637, it is an excellent opamp, but remember that in case of active equalization we need unity gain stable device, unless we introduce additional resistor, that influences HF rolloff. Then further LP pole must be added to get appropriate frequency response.

Hi Pavel,

Have a look at the circuit below. Assume that R is held constant while k is varied. The transfer function of this circuit is:

V2/V1 = [1 / (k + 1)] * [R /(R + Z(s))]

It's interesting because varying k does not change poles or zeros, only the constant out front.

That said, trying an active circuit for moving-coil all in one shot would require lots of low-frequency open-loop gain. Maybe a composite op-amp?

[jcx]

a low output mc impedance requires a low noise riaa feedback network that presents a heavy load at high frequencies so at least a buffer in the loop seems desirable, a cfa op amp also allows large loop gain boost too while driving the feedback network without loading the low noise input op amp and cfa op amps have enough extra bandwidth to simplify loop stabilization

added gain does complicate linear stability and overload recovery but can be dealt with, GHz "GBW product" is possible at audio frequencies (but the roll off above audio has to be faster than single pole)

I have built and measured composite op amp circuits with >90 dB total gain at 20 KHz, ~ 80 dB in loop gain - with much slower op amps than are easily had today

[PMA]

Originally posted by andy_c

That said, trying an active circuit for moving-coil all in one shot would require lots of low-frequency open-loop gain. Maybe a composite op-amp?

Hi Andy,

I did not mean one opamp, but 2 independent opamp stages. 1st is completely linear with gain 20-30dB. 2nd makes RIAA equalization with similar gain, as for MM. This is a well known approach shown by Walt Jung, e.g.