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Old 15th April 2012, 07:46 AM   #1841
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Do you think a common mode choke before the regulator could help? I have a few I could try. Anything else that could be put in between the power supply and regulator? Bigger caps don't seem to affect the hiss much.

Hmmm - I could try a 5 ohm resistor and put the current up. That could work - good thinking. A 12v DC supply would be very handy.

Andy
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Old 15th April 2012, 07:59 AM   #1842
dhtrob is offline dhtrob  Netherlands
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Andy,
I put in a common-mode choke but that did not get rid of the noise. My guess is that the noise is being generated by the used regulator. I had noise with 2 types of VCCS and am using Rod's regulators now, using exactly the same raw supply without noise..
Rob
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Old 15th April 2012, 08:04 AM   #1843
dhtrob is offline dhtrob  Netherlands
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Hurray for Rod and his regulator!!
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Old 15th April 2012, 10:59 AM   #1844
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Thanks Rob!

Yes you are right, the origin of the noise is the LT1085. This chip, and almost every other regulator IC, uses a bandgap reference to control its output level. These works fine for normal industrial applications, but they are just too noisy for high quality audio. Here is the calculation for the approximate noise level for the LT1085, connected as a current regulator:


The data sheet specifies the noise as:

0.003% of Vout, Typical, 25 deg C, RMS value, 10Hz-10kHz.

When connected as a CCS, the regulator applies the reference voltage: 1,2V across a 1,2-ohm
resistor, so the output noise is:

Vn = 1,2V x 0.003% = 36uV rms;
and this noise is translated into current by the 1,2-ohm sense-resistor:

36uV/1,2R = 30uA.

So the CCS injects RMS noise of 30uA into your filament current.

Now, the filament shares its terminals, internal wiring and all with the CATHODE current. So the filament current (which just heats the tube) is DIRECTLY mixed with the cathode-current - ie our Music Signal!

For 26 preamp with the cathode current set at 6mA this 30uA of rms noise is 0,5% of the full current range - and even higher % than this, if we only consider the working range of current of the 26 (the current swing required to play music).

Naturally 0,5% of noise, caused by the filament current is too much. Not only you will hear Hisssss, but the music signal will have sidebands added in the frequency domain (eg signal 1kHz, noise at 10Hz will give sidebands at 1010 and 990Hz - and this
repeats for all the other frequencies in the noise). The sidebands will give clouding of the sound, and spoil the stereo image.

The noise at 0.1 to 10Hz is extra large in bandgaps (and other noise sources), and though you do not hear the sound of it as hum, it is always there, generating sidebands, degrading the sound.


With switching regulators, there is not usually any specification for the **reference** noise, which you can be sure means it is very bad! But the high frequency ripple with switchers will also create sum & difference sidebands, and degrade the sound.

With my regulators, the self-noise is the minimum it can possibly be: just a pair of transistor junctions, whose self-noise is at least 100x lower than a bandgap diode.

This is one reason why regulators made with ICs sound bad. There are plenty of others, such as the way that the Music signal gets mixed up in the feedback loop of the IC regulator, which should only be controlling filament current.

Andy, the old (2004) circuit required higher voltage, but the new PCBs only need 4V (guaranteed), and usually works quite well with 3,6V above the filament voltage.
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Old 15th April 2012, 12:13 PM   #1845
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Thanks for this excellent explanation!

One addition (i read in an old textbook): there seems to be a linear relationship with higher anode voltage in the amplified cathode noise signal. They even have a nice picture.
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Old 15th April 2012, 05:35 PM   #1846
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Wow! Considerable progress here - thanks guys! I now have the 26 preamp running at approx dhtrob's operating points - it's extremely good. 95v on anode, 3.4mA, 5 ohm cathode resistor. It's 4.6v bias running slightly starved filament at 1.2v. So my cheap 12v DC supplies are working just fine!! No hiss.

I'm using two Hammond 156C chokes in series and FT-2 teflon output caps. This is possibly the best 26 preamp I've made yet. Incredible - it was made as a cheap compromise and blow me, it outperforms my other one with 10 ohms cathode resistors, 135v anode and LL1660/5mA. Considerable saving of cost. It's very detailed indeed. This has been a very worthwhile exercise, and it's a blueprint for a very cheap build.

I suspect dhtrob didn't come to this operating point by accident!

Try it one day!! No chokes in the filament supply even. Wow.

Andy
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Old 15th April 2012, 05:42 PM   #1847
mogliaa is offline mogliaa  United Kingdom
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Quote:
Originally Posted by andyjevans View Post
Wow! Considerable progress here - thanks guys! I now have the 26 preamp running at approx dhtrob's operating points - it's extremely good. 95v on anode, 3.4mA, 5 ohm cathode resistor. It's 4.6v bias running slightly starved filament at 1.2v. So my cheap 12v DC supplies are working just fine!! No hiss.

I'm using two Hammond 156C chokes in series and FT-2 teflon output caps. This is possibly the best 26 preamp I've made yet. Incredible - it was made as a cheap compromise and blow me, it outperforms my other one with 10 ohms cathode resistors, 135v anode and LL1660/5mA. Considerable saving of cost. It's very detailed indeed. This has been a very worthwhile exercise, and it's a blueprint for a very cheap build.

I suspect dhtrob didn't come to this operating point by accident!

Try it one day!! No chokes in the filament supply even. Wow.

Andy
Great stuff Andy! I do have my 26 running at 6mA with the LL1660 and 10 ohm cathode resistor. However, I started playing with starved filaments and operating points with the 4P1L, 71a and now the 01a, which is great for me! Love the thoriated filament sound btw.

I will build a 26 with gyrator load and try 95V and 3.5mA to compare results. It would be a great candidate to add a cathode/follower stage to reduce Output impedance as per my previous posts.
Oh no, another preamp on the way!!
Cheers,
Ale
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Old 15th April 2012, 05:56 PM   #1848
dhtrob is offline dhtrob  Netherlands
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Quote:
I suspect dhtrob didn't come to this operating point by accident!
Actually I did experiment with the '26, but was more or less forced into the 95V B+ and -4V Vg1 'cause of limitations on the heater supply..

@ Aje: Rubber stand-offs are arriving this week...

Rob
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Old 15th April 2012, 06:00 PM   #1849
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The 26 in this iteration blew the VT-67, 31 and 01A out of the water. It was really no contest. The 26 is smoother and all round more musical. This 95v operating point is a bit leaner and more detailed with the 1.2v starved filaments. Treble is crisper and it takes out that slightly warm and tubby sound. Well, I don't mean tubby really, the 26 can't ever be that bad. It also restores the balance in my 300b SET - that needed to be a little leaner and more detailed (it has 46 drivers).

I have had great results with my PP amp - currently 01A diff pair direct coupled to 31 interstage coupled (LL1660/PP) to 2a3s. If it's good with that, then it's going to be better still with 26 pairs direct coupled to ??? and LL1660/PP to the 2a3s. So input tube is 26, sorted. Driver should be a pair of 10Y really - I have 46 and 4P1L as options. If the 4P1L works then the gain would be good. This is all looking very promising. I'm going to try out some 6C4C. I'm preferring the old Chinese biplate 2a3s to the Sovtek monoplates, so the old Svetlana biplate 6C4C may be good too. My sound system is really sounding good these days. Pretty happy with my Alpair 10s in 21 litre MLTL. Actually they're infinite baffle right now with plastic bags stuffed in the port.

Andy
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Old 15th April 2012, 07:02 PM   #1850
mogliaa is offline mogliaa  United Kingdom
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Quote:
Originally Posted by dhtrob View Post
Actually I did experiment with the '26, but was more or less forced into the 95V B+ and -4V Vg1 'cause of limitations on the heater supply..

@ Aje: Rubber stand-offs are arriving this week...

Rob
@ Rob, many thanks you are a star!

Andy/Rob,
Just wanted to double check and did a quick plot on my tracer of a NOS RCA 26 ST valve. Filaments starved at Vf=1.2V and 800mA as per Andy's operating point.

Ra increases to 11K (3K more than a 01a) but if Andy reports good bass response I take is worth trying it?
Initially I thought the operating point might be a bit on the low side, but At Vg=-4V and Va=100V is a good linear region. Perhaps a bit marginal at -4V bias to respond to input signal transients and end up driving a bit of grid current?

Anyway, enjoy the plot and draw the load lines
Attached Images
File Type: jpg 26 If=800mA.jpg (118.8 KB, 404 views)
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