Thread: Valve preamp.
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Old 18th Jun 2019, 6:38 pm   #50
Electronpusher0
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Join Date: Jun 2016
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Default Re: Valve preamp.

Quote:
Originally Posted by Craig Sawyers View Post
Just put the RIAA values (without tolerances) from the schematic into TINA. The response deviates quite a lot (+/-0.8dB), with a big hump at low frequency. From 100Hz to 20kHz it is +/-0.15dB.

However, keeping the 330k fixed, take out the 560p and just use 2.2n. Change the 39k to 48.1k (E192 standard value) and the cap in series with that becomes 5.6n//1n (for 6.6n).

That gives +/-0.13dB from 20Hz to 20kHz assuming perfect values
Hi Craig, you really need to model the whole amp not just the RIAA components (I did this in LTspice)
The following is from my design notes:
"The (passive RIAA) circuit is affected by both the output impedance of the preceding stage and the load impedance of the following stage.
• The output resistor of the preceding stage effectively adds to R1’s value
• The input impedance of the following stage is effectively in parallel with R1
Overcoming this problem requires selecting R1’s value such that the combined effects of the preceding stage output impedance appearing in series together with the next stage input impedance appearing in parallel return it to the designed value.
Fortunately, valve amplifiers circuits have significant output impedance. For example, a 12AX7 used in a grounded-cathode configuration with a 150k plate-load resistor results in an output impedance of roughly 44k (62k in parallel with 150k)."
The R1 referred to is the input resistor of the passive RIAA.

I attach an LTspice plot of the whole amp using a reverse RIAA input.

The real world test on the bench confirms this.

Peter
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Last edited by Electronpusher0; 18th Jun 2019 at 6:42 pm. Reason: added note
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