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I'm trying to find what can be done about noise floor in my phono. I have hum down to an acceptable level but now there is noticeable white noise (shhhhhh) which seems higher than it should be. Preamp is a Scott 130 that has been updated with K40 coupling caps, solen ps caps, new bridge rectifier and various other parts replaced as needed. Phono plate resistors are new metal film, phono tubes are Telefunken. The cartridge is a DL-103R going through HK XT-3 SUT's.
The noise is definitely gain-related, it's only really noticeable at pretty substantial volumes but it's quite noticeable then. No such noise at line-level. I've tried substituting phono tubes, it doesn't change anything.
Any ideas of where to look?
Thanks in advance.
Dave
Follow Ups:
5751s are reasonable in the line stage, but the phono stage RIAA EQ is, I believe, done with feedback so substituting 5751 for the 12AX7s would alter the equalization. A number of the old phono stages didn't have very accurate EQ so the substitution could either improve or worsen it depending on how accurate it was to begin with, but in any event it would definitely change it, independent of what it would do to the noise level.
Perhaps the noise is inherent in the design, but I doubt it. I have little experience with Scott tube gear, however I know Fisher tube products very well.
Fisher phono sections in good condition have a slight hiss at full volume (no signal input). The hiss falls off quickly at 3/4 volume. I used 98dB rated speakers.
Perhaps noisy old resistors is the issue, but tubes and dirty connections are the usual culprits. Metal film type resistors are quieter than carbon resistors. As Eli stated the Sovtek 12AX7LPS are quiet tubes. Brimar CV4004 are extremely quiet and unfortunately expensive.
So I had a look inside today. Voltage off of the last filament cap was 18.6V, high but within tube spec. I had a pair of 3.3ohm 2W resistors off of the bridge, a little higher than the 2.7 specified in the schematic. I tried clipping one out, it only brought the last filament cap down to 17.5V, so I clipped the second one out and replaced it with a pair of Mills 12W 3 ohm in series, which measured 6.4 ohm. This brought the voltage at the last cap down to 15.4V, pretty close to the 15V called for, so I left it that way.
I also cleaned all of the switches again, with extra attention to the Mag 1/2 switch.
The bad news is the white noise is unchanged...So I think the next step will be to try 5751s and LPSs.
I appreciate all of the help and suggestions.
Dave
I checked with no phono connected and the white noise is still there, so I think I can rule out the cartridge/SUT as a source though a higher output cartridge may require less gain I suppose.
Dave
David
The Scott 130 has a lot of gain because of so many stages...What I would do to start is to get some 5751s and put them in the phono and the line stage..The tube is a direct drop in for a 12AX7 only it has 30% less gain and I think that will solve a lot of your problem.That's what I did on my 130 and it lowered the tube rush quite a bit.
"If it measures good and sounds bad, it is bad; if it measures bad and sounds good, you have measured the wrong thing."
- Daniel R. von Recklinghausen
If you did change it,measure the heater across pins 4 and 5 of the phono tubes..If it's 12.6vdc,that's too high believe it or not and you may need to put a dropping resistor of a few ohms right off the selenium to get that voltage down to the 10.5vdc range.
"If it measures good and sounds bad, it is bad; if it measures bad and sounds good, you have measured the wrong thing."
- Daniel R. von Recklinghausen
I think all of the 12ax7's are connected for 6.3V, but each channel has the tubes in series. The Scott schematic shows 15v for each of the sets of three, and the Sam's shows 18v. I'll be curious to see what I have there.
Dave
I would trust the Scott schematic. My 299a also calls for 5 volts and that's what I am using. Works perfectly.
Indeed.
"If it measures good and sounds bad, it is bad; if it measures bad and sounds good, you have measured the wrong thing."
- Daniel R. von Recklinghausen
Id go by the scott schematic.You are right,they are in series. I've rebuilt a few lC21s and a couple 130s and I never paid much attention as I usually just tune the source voltage off the filament supply.
You will notice that a pair of 2.7 ohm resistors are in parallel off the diode bridge..Clip one end of one of the one resistor that's in parallel and see if that drops the filament voltage if its too high.If you have 7 or 8 volts across each tube's filament,I would get that down to around 6vdc on the filaments.
"If it measures good and sounds bad, it is bad; if it measures bad and sounds good, you have measured the wrong thing."
- Daniel R. von Recklinghausen
Edits: 02/24/15
I have a Fisher power amplifier out of a console, great midrange, but when I tried 5751 to replace the 12ax7 drivers, there was no output at all. We did a spice simulation and found out the 12ax7/5751's were being operated out of spec. The cause was the power amp contained the power supply for the preamp and tuner, which were not hooked up, so the B+ was higher than it should have been. It was definitely a surprise, since every 12ax7's worked and none of the 5751's.
I'll check that out, I have a friend that can lend me some 5751s to try. I don't seem to have too much gain, I have phono level around 80 and volume at 2:00 to get "loud". Certainly enough but at those levels the noise is an issue in quiet passages. I'll check out the 5751s, thanks.
Dave
Just out of curiosity, have you changed out the selenium rectifier on the DC heaters of the 12AX7 tubes and verified that the heater voltage across each heater is 5 volts as specified by the schematic? You might get a lot more noise if the heaters are off that spec which is a bit unusual for a 12AX7 (I guess Scott liked to starve the tubes for long life). If you still have the original selenium rectifier it may be putting out a whole lot less voltage now (perhaps down to 4 volts?)...and if someone updated it with a silicon bridge without a dropping resistor you might be closer to 7 volts....either of those scenarios might be contributory.
Not something I had considered, thanks. I did replace the bridge, but I'm not certain what the heater voltages are nor had I noticed that they should be so low. I'll check it out.
Dave
At the very least it will spare you from prematurely wearing out a set of TFK 12AX7's
By the way, if you have them, a set of 1950's Raytheon black plate tubes are sensational in the phono position.....even better than the TFK IMHO....and quieter.
David
If its at 12.0vdc or higher on the phono tubes especially,get it down to around 10.6vdc..With the heater voltage too high,this will cause rush because you are really pushing the tube hard.
"If it measures good and sounds bad, it is bad; if it measures bad and sounds good, you have measured the wrong thing."
- Daniel R. von Recklinghausen
Might be the series resistors in the RIAA filter itself, and/or cathode resistors. Probably wouldn't hurt to build an outboard PS with more filtering (chokes) and DC filament as well, if a Scott 130 doesn't already have them. I understand you're not having 60hz hum, but it could help overall anyway IMHO.
I have a 299A which has essentially the same preamp section. I agree with the other posters regarding the switches. Once I bypassed the mag selector as well as the noise and rumble filters, it helped a lot. On my unit, I have to turn the volume to max to hear much noise and it isn't much.
With the stock set up using 12AX7 tubes, the stage seems happiest with a moving magnet cartridge with an output of ~5mv. I've used a Shure VxMR at 3mv but the gain of the phono stage is a bit borderline for that. I am wondering if your problem is related in some way to your SUT setup and the interaction with the amp.
By the way, I am not using fancy tubes in my phono stage....just plain JJ ECC803S. No hum.
Good info, thanks!
Dave
Fact: S/N in phono is poorer than line level sources, especially digital. You may be running into some fundamental limits.
1st, buy a pair of culled phono grade 12AX7LPS tubes from AA sponsor Jim McShane. 2nd, clean your switches with Caig DeOxit. You may get adequate relief by installing superior noise factor 'X7s and switch cleaning.
If, as I fear, top notch tubes and switch cleaning do not provide sufficient relief, reengineering may be needed. Do you exploit the multiple EQ capabilities of the unit? If not, as is so frequently the case, getting switches out of mV. level signal lines is very much in order. A single pair of RCA jacks hard wired to the low level mag. preamp permanently configured for RIAA is the way to go. All source selection occurs at line level, where it is less likely to cause grief. Fewer mechanical connections in the signal path NEVER hurts.
Passive EQ phono sections exhibit better overload behavior than active EQ setups. The tweaked RCA setup I've uploaded exhibits superior bass extension and it REALLY can drive the recording O/Ps. ;> ) A little TO92 case LR8 regulator in each channel takes care of the 250 VDC requirement and makes for good channel separation.
Eli D.
great post and a lot has to do w/ two gain stages (phono and line) but your hints and recs are right on. Thanks for helping make AA as good as it is. Perhaps too he has a modern very sensitive high z in power amp that can make noise floors easier to hear. I don't know the gain of that pre, couldn't find it but when my 27db gain CJ was plugged into a modern sensitive power amp it was noisier than I'd like.When I went to a passive linestage I got a 3/12AX7 phono pre w/tube regulation and passive/switchable EQ and used a high output Grado and that was really noise free, the quietest phono rig I ever had.
E
T
Edits: 02/24/15
I use both modern SET and vintage PP monoblocks, depending on the day. Neither are unusually sensitive in current configuration, and the white noise issue is the same on both. I may try switching out the Denon for a Grado Gold to see if there's a difference, and there are some other suggestions here to try, I appreciate the help.
Dave
Good luck! You'll get it but really noise floor is just something that is a bother when there is nothing playing. You can't hear the hiss while music is playing right? If so something is up. And yes try a high output cart too.
E
T
It's not audible when any real sound is happening in a recording but it is plainly audible in quiet passages or pauses, so it's a drag in subtle music and stuff with wide dynamic range. I'm quite curious to look into heater voltages, should get the chance tomorrow night.
Dave
Thanks Eli. I've cleaned the pots thoroughly. My expectations in terms of noise floor are in relation to other systems I've heard. While it's tough to measure accurately I'm getting something around 40dB difference between the music level and white noise - 95dB music and 55dB white noise or so on an SPL meter - and the white noise is significantly louder than any hum. I usually don't listen anywhere near that loud, and from the listening position it's lower, but it's clearly audible and I've heard significantly better from other vintage setups and I'm suspecting something is not quite right - cathode resistors? Bypass caps? I will have a look into the lps tubes as well. I do use the different eq's and I like that about the 130, but perhaps I'll try to build your RCA phono as a stand-alone, I've often thought about doing that at some point.
Dave
Clean all the SWITCHES, rotary and slide. Dirt in mV. level signal lines increases resistivity and makes for noise.
Eli D.
Ok, I believe I've done them all but I'll go through it again, thanks.
Dave
Dave
On the back you have preset volume input controls..Set those as low as you can so you can run the volume control in the 10 O'clock to 1 O'clock range where it is most linear.Also,be sure that your left and right phono input jacks on your 130 are across from each other and not below each other as the other jacks are for aux,tuner,tape,etc. Mag1 jack are on the top left and right and Mag 2 jacks are on the bottom left and right.
If you continue to have hiss,try going to a 5751 which is a lower gain tube or the 12AX7LPS which has a spiral filament.
"If it measures good and sounds bad, it is bad; if it measures bad and sounds good, you have measured the wrong thing."
- Daniel R. von Recklinghausen
Thanks Mike. I've tried the phono level pot in different positions, the white noise seems to relate to the gain/final volume irrespective of what combinations of settings I use between the level pot and volume knob. I'll probably check into the 5751 or lps. Would 12au7 be worth trying?
Dave
You can't use a 12AU7 in the phono because it won't load properly being the plate resistances are vastly different and you won't have enough gain and then your RIAA curve will be wrong as well..You would have to redesign the whole circuit..
"If it measures good and sounds bad, it is bad; if it measures bad and sounds good, you have measured the wrong thing."
- Daniel R. von Recklinghausen
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