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In Reply to: Best inductors for RIAA? posted by chaslieb on October 27, 2001 at 21:10:26:
i think you will be best avoiding the air-cored inductors for the LCR riaa... the linearity of them is really desirable, but even a value of 45mh would be pretty big, and 1.8hy?? while possible, i think you may be looking at a 10 pound spool of 15 ga wire for starters :-)like all riaa eq's its best to cut and try, and in this case its a wind and listen situation... i suspect if you really want to get into it, and follow through, you will have the best luck winding your own, or be prepared to pay for lots of different 1.8hy inductors... you cant simply call a winder and spec it and expect the best results... inductance values are very soft numbers... and the riaa requirements may or may not work well when you plug in a soft number...
the two values you state are quite similar to the tango eq-600 numbers... which they publish... if it were as easy as putting all those parts into a can... they wouldn't tell you whats in there... i have seen numerous people try to duplicate this circuit, and one of two things happens... they try it once, and it fails and they proclaim LCR EQ sucks, or they try it once, again it fails, but the see promise and spend another year on it and end up with something brilliant.
sadly very few people fall into the second category...
dave
Follow Ups:
The Tango LCR RIAA box is indeed a frustratingly difficult thing to re-create without MUCH suffering and heartache.After considerable anguish and penance, I'm going to an AV20 - Magnequest DS-050/600 - Tango LCR RIAA and try to have fun with phono again.
Hi Dave, All,I guess the reason why people failed to duplicate the Tango
RIAA is because of the DC resistances of the coils. I spent
quite some time on LCR RIAAs and carefully analyzed Tango's
design. They did a fine job of compensating the DCRs of
the coils. That's why they put in those parallel resistors
to the caps. And that's also the reason why they use 1.8Hy
instead of 1.9Hy which the calculation gives assuming 0 DCR.Just dubbing the tango circuit with soem coils with different
DCr will of course yield a different result.I tried to put the formula for a LCR EQ with DC resistances in
an Excel sheet. Anybody interested in this can email me. But
be warned, it requires at least some basic knowledge of the
subject, it's not foolproof and will give rubbish results if
the inputs are not ok.To my ears LCR EQ is sonically the best. I havent' heard
a LR EQ yet. I'll probably give that a try some time.Ciao ... Thomas
Have you built your own LCR RIAA preamp? If so, do you have a schematic with parts list? I would like to build one but the concerns about the dc resistance of the chokes and impedences as well as static resistances of interstage transformers leading to the stage has kept me away from trying. Thanks for any help.Charles
Hi Charles,So far I have built 5 Phonostages with LCR RIAA. I have no
schematic. They are so simple that's not necessary to draw
them. All of them are transformercoupled. Three use the
Tango RIAA and are based around the Telefunken EC8020.
Unfortunately this tube is very difficult to get. But there
are alternatives. One of the other two is an all DHT preamp
with 801As. For this one I built a 'discrete' LCR RIAA.
The last one was an experimental phonostage with 6HV5.
For this I built a RIAA with UTC variable inductors. This
one led me to the analysis of the Tango RIAA. Obviously
the DC resistance of the UTC coils is very high which for
example leads to a early bass roll off.I will try my calculation scheme with those UTC coils to
see if it's valid.
Typically I drive the LCR with a step down transformer,
so that it's fed with a source impedance of 200 Ohms or
less. The DC resistance of the driving transformer is not
critical. It will only generate some loss, but it's not
frequency dependant.To get enough gain I always place a 1:4 step up transformer
after the RIAA.There is a schematic of a LCR phonostage in the latest
VALVE magazine. The article is from a friend of mine.
His Phonostage is very similar to my EC8020 based
one.Regards ... Thomas
Thanks for your response. I was considering building the RIAA in the latest "Valve" but was unsure whether the inductors used had a specific static resistance that was critical to the design. You note that in your own "discrete" design the high resistance of the inductors caused bass rolloff. Is there any guideline to use to determine what the total resistance of the circuit should be from input to output, or any simple way to compute the inductance as compared the resistance of the inductors and how resistor and capacitor values are dependant on the inductor values? Also, how do you find the TANGO Eq-600 works in your circuits? I tried to buy a pair a year or so ago and was put on temporary and then permanent hold when Hirata closed the company. Don't know where to look for them now so just trying to do it on my own.Thanks again,
Charles
Hi Charles,as mentioned in my previous post, I put the formulas
for the LCR RIAA into an excel sheet. You enter the
corner frequencies you want, the inductance and resistance
of the coils and the sheet calculates all the resistor
and capacitor values.The inductance values need to be close to the nominal
values, otherwise the sheet will produce rubbish. This
technique to compensate the DCRs of the coil is an approximation.
I haven't tried for real high DCR coils yet.With this sheet you can also calculate the LCR values
for other EQs than RIAA. You can also put in the 'secret'
4th timeconstant 3.18us a la Allen Wright.The sheet generates values for two different kinds
of LCR network. One is the Tango-Version. The
other one has a slightly different resistor arrangement.Email me if you want the sheet and I will send it. I'll
help you to come up with the right values.Although I like the Tango units very much (I use them in
my main phonostage) I think they have been over-mystified.
There is nothing magic in them it's just solid engineering.Ciao ... Thomas
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