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In Reply to: Re: Best inductors for RIAA? posted by Thomas Mayer on October 29, 2001 at 02:42:28:
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.
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
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
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.
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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