Converting an Apt Holman preamp to tape playback

SeleniumFalcon

There is a similarity between the necessary gain requirements and the equalizations needed between RIAA phonograph playback and NAB/IEC magnetic tape playback that should allow a well designed phono circuit to be used for tape playback. Many older preamps (Dynaco PAS2) and receivers (Fisher 500C) had front panel selectors to switch between these sources. My goal was to use a top performing phono preamp and change its circuitry to allow for tape playing.
My recently acquired Apt Holman preamp looked like a great opportunity. It uses an FET/BJT differential input and fantastic RIAA accuracy and a very thorough service manual so I set out to convert RIAA to NAB. The RIAA has three poles in its EQ design versus two poles with NAB so one of the poles was bypassed plus the high frequency rolloff had to be extended slightly. After some research and calculations I think I've got it.

audioluvr

I liked this because I have a lot of respect for someone who has a passion for restoring old technology. No, I have no idea what you are talking about, but I've done similarly complex projects and have enjoyed the challenge immensely!

Keep up the good work!

newbie308

That is going to be a one of a kind, amazing deck when you are done! Nice work!

SeleniumFalcon

The RIAA EQ has three poles or frequency ranges where the curve makes changes:
Reducing at 6dB/octave starting at 50Hz, then leveling out at 500Hz and dropping again at 2kHz.

SeleniumFalcon

The NAB (tape) playback curve doesn't have the 500Hz plateau:



It begins rolling off at around 50Hz at 6dB/octave until around 3.2kHz where it begins to move upwards.

SeleniumFalcon

Here is the schematic drawing for the phono section of the Apt Holman:



What is happening is the first pole of the RIAA circuit is created by the 100uF capacitor C13 in the lower left area. This is acting as a 6dB/octave low pass filter at around 47Hz.

SeleniumFalcon

At the top of the schematic there is a feedback loop created with two capacitors C15 and C16 that are paralleled with resistors R16 and R17. They take the output from transistors Q6 and Q7 and feed it backward to the pair of input transistors Q1 and Q2 but since they are tuned to two different frequencies by their resistance and capacitance they pass some frequencies and reduce others resulting in the shelf at 500Hz and the dropping at 2100Hz.

SeleniumFalcon

One of the things I found out was that C13 and C113 (left and right channels) was off the required values. Instead of being 100uF they were 135uF which would push the low frequency response off of the RIAA curve. I replaced them with good old reliable Elna Silmic II 100uF capacitors.

SeleniumFalcon

So I could keep the 50Hz low frequency roll off in place but I had to eliminate the shelving at 500Hz and change the last pole from 2.1kHz to 3.1kHz. This would allow the two responses to be more similar.

pitdogg2

The Elna Silmic II's are wonderful caps.
I've used them many times. Smooth as silk.

SeleniumFalcon

I still wish the old Black Gate capacitors were being made, or even the original Panasonic series were good sounding. The Apt Holman uses some fairly standard black covered capacitors that were very common for that era. If I were building the circuit from scratch I'd use some huge WIMA 50uF in parallel since this is an important component.

pitdogg2

I've used with good results Nichicon Fine Gold. Although I've not used them others I trust speak highly of the Nichicon muse KZ series. Have you played with them?
The problem may be that the values are limited.

SeleniumFalcon

I haven't had the opportunity. The phono preamp I use has the Nichicon Gold as filter capacitors in the power supply. I believe they were chosen by Tom Tutay, the designer. They have a fine reputation, where have you used them?

pitdogg2

Power Supply of my Sony X33ES CD player.
Here's the thread

https://forum.polkaudio.com/discussion/127903/can-i-join-the-club#latest