Phono EQ hiss/white noise!!!

[chanwoo]

Recently I built LC passive RIAA EQ for MM cartridge and line stage as well. However I have excessive hiss/white noise from both of phono and line stage since I'm using Altec horn speaker. I used 2 of 12AU7 for all stages and AC heating for them(AC hum is acceptable). I'm guessing the first phono stage is genarating most noise because if I ground second phono stage grid the noise level was acceptable.

Especially I added 100Kohm between Gate-Plate to reducing phono gain.(Attached circuit diagram) Can you tell me what is wrong in my circuit? I tried replacing some of resistor but change nothing. And also I increased Ra on first phono stage from 47K to 100K to reducing current but change nothing as well. Is this somehow oscillation???(Sha~~~

Thank you in advance. I’m going crazy.

[analog_sa]

I don't think this qualifies as an LC EQ. Why is there an input transformer if you use it with an MM cart? Reducing the input stage gain is not a good idea as far as noise goes. Another possible source of noise is the PS which is not shown.

[padamiecki]

Hi
should you add a capacitor in series 100k nfb resistor to block dc?

[Rod Coleman]

Some grid stoppers are recommended - 5K to 20K carbon composition preferred, but 1206 chip work well too. you may get oscillation without these. Put them on the valve base, don't use long wires.

Be sure the heater is 30V..60V above the cathode. Use voltage divider resistors from supply voltage. You get hiss and crackle from leakage current if you don't have this.

what is transformer ratio?

[chanwoo]

To Analog_sa: My bad. It is not LC EQ but RC passive EQ. I'm suppose to utilizing the input transformer as far as if it's not related the noise generation because it was there before I intergrated phono. And for the PS, I used CR pie filter like Diode-47uF-50ohm-47uF-5Kohm-47uF... There is no filter chock because small space. And I used 6VAC for heaters. It can be change to DC once I succefully reducing hiss noise.

padamiecki: Good point. You are right but pretty strange it is working now. And this may not related noise because I had put this resistor to reducing gain for purpose of noise reduction. I will put C in the series of 100K.

Rod Coleman: I'm not cleary understanding the place to put such a low value of R and meaning of 1206 chip & valve base??? Do you meaning use tiny SMD resistor and put it inside of tube socket(Round hole)? I'm using 6VAC for tube heater and have 200ohm hum balancer and grounded it's center. Do you think I must use heater floating as your described to reducing hiss noise? I thought this technology is only for SRPP to keeping H-K voltage within specification. The transformer ratio is 1 to 1. It is a German transformer and has pretty good frequency response.

[chanwoo]

I just applied heater floating to 50VDC but this noise reduction performance seems worse than grounded. The heaters of 12AU7 for phono and line stage are tied together. Any more action items?

[Allen Wright]

In my opinion, your whole gain structure is wrong, and I'm not surprised you have high noise levels.

Mostly you have **THREE** 12AU7 stages after the RIAA network, all adding their noise sequentually.

The 12AU7 is about the last tube I'd ever use for phono, being low gain but very high noise.

A pair of 12AX7 triodes or a pair of 6DJ8 triodes will give you all the gain you need for MM, without the input & output transformers. And FAR FAR less noise.

Simply moving the RIAA network to the end of this string of tubes will make a big difference. I would never use more than one low gain tube after the RIAA, and preferably none.

Regards, Allen

[Rod Coleman]

You can be sure Allen is right.

you could modify your circuit like Thorsten's easy to build version:

dc phono

[chanwoo]

Many thanks to everyone. I'm still learning tube audio design because I'm inquisitive guy and this is one of my best hobby as well.

This pre amp is mono block. I better redesign phono amp using single ECC83 or single ECC88. Why use single tube? Because no spare heater supply.

Here is another question for you. Sometimes the amp I designed has excessive gain and I'm always added NFB to reducing it. The NFB is correct solution? I don't like NFB. What other solution to reducing gain of amplifier?

Thank you.

[Rod Coleman]

Originally Posted by chanwoo

Here is another question for you. Sometimes the amp I designed has excessive gain and I'm always added NFB to reducing it. The NFB is correct solution? I don't like NFB. What other solution to reducing gain of amplifier?

Thank you.

Use fewer valves (tubes) if you have too much gain. And remove the capacitor across the cathode-resistor.

When you design an amplifier, think about gain before anything else, and choose the valve that will give what you need.

Triode gain-stages don't need NFB, and it may ruin the sound! Pentodes are different, and sometimes NFB is OK.

Try some of the links in the Sticky-thread to find out about gain, NFB, and tube design generally:

On Line Tube Learning for newbies....