Fisher 500tx phono section

[AK577]

How are the phono stages in these era fishers? I recapped the phono board in my 500tx and it’s still lifeless compared to others. Is that expected?

The high-low switch on the back, I have it on “low” I believe, which is louder. When it is set to high, it’s pretty quiet.

Turntable is a dual 1219 with Shure V15 III. It sounds amazing with my Marantz 2285b, so I was excited to try it on the fisher and I’m pretty disappointed.

Anyone have similar experiences or suggestions?

Thanks!

 

[wparks]

I found the service manual for this model at Elektrotanya. Referring to the schematic for the phono pre-amp, the "high low" switch is working correctly. Low is for low output cartridges (2.7mV), so will be louder. High is for 10.7mV cartridges.

The phono pre-amp with RIAA equalization is not very good in this amplifier. The phono input is =heavily= loaded at 680pF, which I would expect no loading given you get ~100-150pF in your phono RCA cable. It is a very pedestrian two transistor design, with all of the RIAA correction in one big lump, and is shipped all over the chassis, so probably noisy. Transistor pre-amps went out once op-amps arrived offering much better performance. Time for an external RIAA phono pre-amp.

If you are into building projects, I can most highly recommend Rod Elliott's project #6, which I use, and is stellar. Virtually any cheap op-amp based design will run circles around this built-in unit, but if you can, shoot for one a little more expensive, for which you can probably find a million recommends around here.

 

[gadget73]

I sort of wonder if the funky loading on some early phono stages is actually to compensate for some frequency response issue. Possible overall it comes out OK, I really don't know but a lot of early SS have odd resistive and/or capacitive loading. I'd have to figure by the late 60s engineers would be familiar enough with phono cartridge loading to not pick wrong values for no reason.

 

[wparks]

Really depends upon the needs of the cartridge. I'm sure the engineers did the right thing for the popular cartridges of the time (or maybe Fisher turntables?) but there have been a lot of variations, and misunderstandings in this area.

I am most familiar with moving magnet, the most common modern type. There is an excellent article by Rod Elliott on the MM cartridge and loading that provides probably way more technical detail than most can stomach. Long story short, the more capacitance loading of the cable + phono input, the lower it moves a resonant frequency into the upper end of the audible range. He suggests many engineers misunderstood that when the cartridge manufacturer asked for say, 100pF loading, that was typically provided by the cable, yet many still put a discrete 100pF or much more loading into the front end of the pre-amp. Maybe they were expecting zero capacitance cables? No idea.

I measure the tip to shield capacitance using a very cheap Chinese meter, and only use phono cables in the 100-150pf range. (Testing my cables, some were up in the 300-400pF)

 

[AK577]

I found the service manual for this model at Elektrotanya. Referring to the schematic for the phono pre-amp, the "high low" switch is working correctly. Low is for low output cartridges (2.7mV), so will be louder. High is for 10.7mV cartridges.

The phono pre-amp with RIAA equalization is not very good in this amplifier. The phono input is =heavily= loaded at 680pF, which I would expect no loading given you get ~100-150pF in your phono RCA cable. It is a very pedestrian two transistor design, with all of the RIAA correction in one big lump, and is shipped all over the chassis, so probably noisy. Transistor pre-amps went out once op-amps arrived offering much better performance. Time for an external RIAA phono pre-amp.

If you are into building projects, I can most highly recommend Rod Elliott's project #6, which I use, and is stellar. Virtually any cheap op-amp based design will run circles around this built-in unit, but if you can, shoot for one a little more expensive, for which you can probably find a million recommends around here.

This answers all of my questions! Thank you so much

 

[gadget73]

Wonder if it duplicates the Miller cspacity of tube phono stages. Those are fairly high so it doesn't need an extra cap.

 

[fred soop]

From the 700-T forward, there appears to have been a lot of parts reduction effort. The later preamps did not have a separate phono preamp. It was all the same preamp and high level signals were attenuated to low levels, then reamplified. Very bad for increased noise level.

 

[gadget73]

that sounds like a terrible idea. I'm assuming in phono mode it switched in the EQ network and in high-level mode it subbed that with an attenuator ?

 

[fred soop]

that sounds like a terrible idea. I'm assuming in phono mode it switched in the EQ network and in high-level mode it subbed that with an attenuator ?

Yes, the EQ was switched in for Phono and Tape Head. But look at the Aux input. The high level goes through a 220K resistor to the low level, then another 220K to the actual input with a 4.7K to ground. That's a voltage divider of about 94X or a 1 volt input is attenuated to 10mV, then eventually reamplified.

 

[gadget73]

crazy. Yeah I bet that does make for a very noisy unit.