Hi. I recently aquired a Threshold SL-10 and noticed a slight rushing noise on the right mm phono channel that gets louder as I turn up the volume. Left side is free of noise/rushing sound.
I decided to recap the entire unit with Elna Silmic II and Nichicon Muse KZ. I also put new big 10000uf Mallory caps in the power supply. It is now sounding very good but the rushing sound hasn't changed. Any idea what to check or replace?
I decided to recap the entire unit with Elna Silmic II and Nichicon Muse KZ. I also put new big 10000uf Mallory caps in the power supply. It is now sounding very good but the rushing sound hasn't changed. Any idea what to check or replace?
An externally hosted image should be here but it was not working when we last tested it.
If the power supply rails measure good, maybe a failing transistor. Whats the DC offset on the bad channel?
some info here : Treshold Treasure ?? while Papa was there : SL10 Preamplifier | Zen Mod Blog
rushing noise - without MC stage engaged ?
rushing noise - without MC stage engaged ?
I measured DC offset at the recording output and it's pretty near zero. Voltage coming from the power supply is 23v. I am getting 14.25v on the ~15v regulated output.
I only used the mm input so far. I will try the mc input later and see if I hear the same noise. Line section is noiseless so I think the problem must be in the phono section.
no need to try MC , if it's already excluded - culprit is in that channel's MM stage
first check for cold solder joints ;
proceed with replacing servo OP ; if not , put back old one
then start replacing transistors
it would be nice to have Hfe matched ones for input LTP
first check for cold solder joints ;
proceed with replacing servo OP ; if not , put back old one
then start replacing transistors
it would be nice to have Hfe matched ones for input LTP
I am reflowing some solder joints and will try again. The OP looks like an RC4136N and there are five MPS6571 and one FPN4250 transistor in the phono. I will try replacing some parts and report back on my findings...
Just an update. I examined the board with a magnifying glass and resoldered any joints that looked suspicious but didn't find much improvement. I removed, rearranged and resoldered some of the 100k resistors near the opamp chip and things got noisier on both the phono and line stage. Gain seems much higher for some reason as I can only go about a quarter turn on the volume now whereas I had it around half way before. At least the noise is about equal on both channels now. I am suspecting noisy resistors or opamp but still not sure.
I ordered some parts: new 100k resistors, RC4136N opamp chip, both 7815C regulators, MPS6571 transistors and 4250A transistors plus a few other things.
I will start with the opamp chip and resistors on that end and see if any improvement.
I ordered some parts: new 100k resistors, RC4136N opamp chip, both 7815C regulators, MPS6571 transistors and 4250A transistors plus a few other things.
I will start with the opamp chip and resistors on that end and see if any improvement.
The opamp is just a DC servo, and it would have to be actually broken for
the noise to get through its 100K output resistor.
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the noise to get through its 100K output resistor.
😎
old transistors in the phono section can get noisy; especially if they been zapped by static electricity a few times over the years.
i agree with ZM suggestion to change transistors.
mlloyd1
i agree with ZM suggestion to change transistors.
mlloyd1
Sometimes you can heat the top of a transistor with the tip of your soldering iron and listen for changes in the noise. Don't heat too much!
A freeze mist spray will work sometimes but this is something that is not usually in the diy users toolbox.
A freeze mist spray will work sometimes but this is something that is not usually in the diy users toolbox.
An inverted aerosol can duster works well. I have cooled down overheated ion pumps using this method.
I replaced the opamp and the nearby 100k resistors just because I had the parts. Also checked and re-soldered the caps by the opamp and the ones near the regulators. I used a flux pen on the joints this time to make sure everything was soldered well. I think I may have had a poor joint in one cap near the opamp which is why I had some hum that came and went around that area.
The phono stage still has some mild rushing noise above half way on the volume dial. Is this considered unusual for this preamp? I can hear some faint noise in the line stage if I crank the volume to 100% but this is way over any normal listening level.
I think the next step would be to change out some transistors as Zen Mod and others suggested. I have the 4250a's but I am still waiting for the MPS6571's to arrive. I accidentally ordered two 7815 instead of a 7815 and a 7915 so I will not replace the regulators until last resort. 🙂
The phono stage still has some mild rushing noise above half way on the volume dial. Is this considered unusual for this preamp? I can hear some faint noise in the line stage if I crank the volume to 100% but this is way over any normal listening level.
I think the next step would be to change out some transistors as Zen Mod and others suggested. I have the 4250a's but I am still waiting for the MPS6571's to arrive. I accidentally ordered two 7815 instead of a 7815 and a 7915 so I will not replace the regulators until last resort. 🙂
The question is, is the noise about the same on both channels? If so, it's
very likely the performance you should expect from the setup you have.
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very likely the performance you should expect from the setup you have.
😎
Thanks Nelson, good to know. I am convinced that the line stage is working as intended. It sounds very clear and energetic with quite punchy bass and a nice forwardness in the mids. Noise is not a problem even at a pretty high volume level.
I am still hearing some rushing on the right side phono that seems unbalanced with the left side which is quieter.
The channels sound balanced as far as loudness goes. I turned it on today and the noise situation seems better with just some gentle rushing on both channels of the phono at very high volumes. I think the high gain and resolution of my Ergo amp2 headphone setup was making the issue seem worse than reality and it's possible the headphone amp was a little out of balance. (it has very tweaky balance potentiometers on the back) Like Mr. Pass said this may be about normal operation at this point. The sound quality is excellent since the last round of re-soldering.
I may replace the RCA jacks as these ones have a tendency to short briefly if the plugs are rotated/twisted.
I may replace the RCA jacks as these ones have a tendency to short briefly if the plugs are rotated/twisted.
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