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-   -   Why is my phono amp fatiguing? (http://www.diyaudio.com/forums/tubes-valves/109540-why-my-phono-amp-fatiguing.html)

duderduderini 5th October 2007 03:23 AM

Why is my phono amp fatiguing?
 
Hi Guys
well i have tried everythingfrom 4 different cartridges (mm and mc) and 2 different head amp topologys as well as stepup transformers.
Its the cornet octal based on Jim Hagermans 6sl7 6sn7 design. The line stage is srpp 6sn7 and it sounds very nice indeed. Cd's currently sound way better than the record side of things.I Suppose my question is ... is it the 6sn/sl7 tubes that are not suitable?
I wonder if i could add some feedback or will that affect riaa?
If i can do feeback then how is it implemented.
I wont post a circuit because its copyright but having said that I downloaded it from here.
Has anyone had any experience with 6sn7's working well for phono.?
Any help will be appreciated
Many thanks
Nick

richwalters 5th October 2007 04:23 AM

Re: Why is my phono amp fatiguing?
 
Quote:

Originally posted by duderduderini

and it sounds very nice indeed. Cd's currently sound way better than the record side of things.I Suppose my question is ... is it the 6sn/sl7 tubes that are not suitable?
Has anyone had any experience with 6sn7's working well for phono.?

Fatiguing ? Define the problem closer, loss of level, quality etc. In my case can't rule out the pecularities with my own hearing.
Tube fatigue can imply many things, loss emission, B+ problems and so on down to component level.
In the Radiotron handbook there are many down-stream circuits appro for phono using sim tubes for phono. Both 6SN7 and 6SL7 were used in many 1950'-60's front end circuits and are fine time served tubes. Don't blame them.

Conrad Hoffman 5th October 2007 04:25 AM

I'm seeing something similar in my solid state world right now. I just put in a new cartridge and got the alignment better than I probably ever have, and it's too crisp. IMO, the rather old records I'm fooling with probably *are* fatiguing, and sounded better with the edges knocked off. What are you playing? Also, have you built an inverse RIAA box and checked the curve? Hint- nobody will be the wiser if you alter the curve slightly to suit your tastes.

Shoog 5th October 2007 07:55 AM

Phono circuits can be prone to radio frequency oscillation. This manifests as Intermodulation distortion - which produces a hard to define high frequency fatiguing quality. Superficially the music sounds fine.

Of course most of the cheaper cartridges are excessively bright sounding and have their own fatigue issues.

Shoog

duderduderini 5th October 2007 12:12 PM

SHoog has hit the nail on the head
 
Shoog bless you for describing it. The cornet octal phono sounds great but i get this hash in my ear and then tinnitus for a day.. its a peculiarity of my ear..it doesnt happen with cd;s movies etc.. I am sure its the phono stage. Itg is particularly apparent on voives in the upper register and lead guitars for instance.
I am running a regulated power supply and again if it were b+ it would manifest in the line stage would it not?
I love the 6sn/sl7 and I am not blaming them or Jims design for my woes.. its just that i cant at least to this point put my finger on it as to why it does it.
So how do you deal with RF oscillation?
Nick

Shoog 5th October 2007 12:31 PM

If you haven't got grid stopper add them right on the pins. If your anode load is not mounted right on the pin - move it to the pin or add 10R anode stoppers. Screening cans on the input valves are designed to deal with radio interference.

Another potential problem could be transformer/choke ringing. Excessive capacitive filtering can cause this and will result in radio waves been transmitted inside the case. If using SS rectification - snubbers across the diodes, and a small bit of series resistance in line with each diode will help kill switching noise.

Shoog

duderduderini 5th October 2007 02:29 PM

will do
 
Hi Shoog
will do as suggested.
My psu is tube rectified clc then regulated via 6080 12at7 and 5651 outputing to a crc filter. This is a seperate psu to preamp.
I may have used a few to many caps in phono stage however and will check'
If there is no grid stopper in original schematic then I add one
right? What value (same as anode stopper?)
Much appreciated
Nick

kevinkr 5th October 2007 04:16 PM

I have run into a number of 6SL7 derivatives that IMHO don't sound that good unfortunately, in particular the standard Russian version and the industrial 6188. You might want to do a little tube rolling as I am quite familiar with several of Jim's designs, and this should not be fatiguing to listen to in the least. Cartridge loading if you are using a high output MC cartridge might also be an issue.

Grid stoppers should be in the 220 - 2.2K range for the 6SL7 family, I've not found anode stoppers to be necessary with tubes in this transconductance range.

Is this a Hagtech pcb or something you wired point to point?

This design is quite revealing and choice of coupling caps and equalizer section caps is critical.. Please tell me you are not using mica here. (There are some Russian micas that might be the exception.)

Another thought is the cathode bypass caps if used. I recently designed a new phono stage with D3A in the first stage and used cathode bias as leds weren't practical for the low bias voltage required. (1.2V/ 20mA) I used Black Gates, but needed large values to assure a flat frequency response below 20Hz - in the end I had to shunt the caps with a 5uF low voltage film cap to get rid of a high frequency glare that defied resolution. Cheaper caps (Nichicon Muse) generated audible and scope visible bursts of distortion on dynamic material - and it wasn't bursts of oscillation in the D3A, it turned out to be a d/a related effect, and was harmonically related to the signal applied across the cap - sounded awful.

You can't use negative feedback in this design, and it frankly shouldn't be necessary.

Do you have a scope to check for oscillation issues in your voltage regulator? (I use somewhat similar designs in all of my projects.) Check the pre-amp as well.

Finally depending on arm and table you might want to consider one the current Grado Prestige series cartridges, even the least inexpensive models perform well and are quite sonically inoffensive in a variety of situations.

duderduderini 5th October 2007 04:33 PM

Hi Kevin
 
Hi
I am running Brimars in all sockets.. nice tube.
No am not using mica.. some are polypropylene made by wima or sangamo oil in paper
Jim has designed in 220 r grid stoppers.
I have just spent 2 hours getting all the resistors close to the pins.
I have point to point wired it, I didnt know you can get pcb's for the octal version of his phono amp.
I have a scope but havent used it. I suppose the fact that the line stage section of my preamp (srpp) sounds so good led me to assume the psu is ok (correct me if I am less than right).
There are no bypass caps.
Lastly i have just ordered an Ortofon MC25FL.. too late for a grado unfortunately.
I will see if the rf detailing that shoog suggested bears any fruit.
I stress again I am v impressed with Jims design. I believe I am getting some form of harmonics/ distortion superimposed. No doubt the scope will show that.
Well its 3am here in Oz and in true Geek tradition I will soldier on and solder on till the sun comes up.
Nick

Shoog 5th October 2007 05:54 PM

Nick,


Quote:

I have a scope but havent used it. I suppose the fact that the line stage section of my preamp (srpp) sounds so good led me to assume the psu is ok (correct me if I am less than right).
Unfortunately you may be wrong. Because the input signal is so small, and the due to the nature of some cartridges any radio pick up/oscillation may constitute a large proportion of the input signal which is been amplified as a whole. In the case of a line amp this is not the case and such artifacts may be swamped and go completely unnoticed.

Again, unfortunately, it may be extremely difficult to scope out a power supply for instability because your not just dealing with voltage - but current as well, and this doesn't show up on a scope. I used to think that simply throwing bigger inductors and caps at a power supply could solve all problems. Recent reading and experience have led me to believe that quite the opposite might be the case. Also - not all regulators sound nice (though i would guess that your tube based implementation probably sounds excellent).

Good hunting.

Shoog


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