A Very Compact Hybrid Amp

[helzerr]

Is this CTH (equipped with 12BH7A) noise floor as measured by RMAA typical?

While I understand voltage doubler supplies like the one used for CTH's B+ may produce this sort of noise; with only ~2 mA load and capacitance multipliers I wouldn't expect this much 120 Hz and harmonics to be present at the output. Perhaps the heater supply is responsible? Thankfully, none of this noise seems to be audible through my 'phones.

Note, this amp is built with R18=100Ω and the 330 μF C3H update installed.

I tried another 12BH7A with nearly identical results. RMAA testing the DAC (y1) and another amplifier (Mini3) yield virtually no noise...

In my quest to build the quietest CTH on the block, the case is grounded to the SG point and all internal signal wiring is shielded, with each shield connected to its respective signal ground at only one end of the cable to prevent ground loops - I.E. input wiring shields are tied to IG at the volume potentiometer, output wiring shields tied to OG at the output jack, and power wiring is twisted-pair...

 

[helzerr]

For comparison: RMAA results at 48 KHz / 24 bit for AMB Labs Mini3 with the same DAC (y1), same cables, same sound card performing A/D duties (M-Audio Revolution 7.1):

 

[cfcubed]

Quote:

Originally Posted by helzerr /img/forum/go_quote.gif While I understand voltage doubler supplies like the one used for CTH's B+ may produce this sort of noise; with only ~2 mA load and capacitance multipliers I wouldn't expect this much 120 Hz and harmonics to be present at the output. Perhaps the heater supply is responsible? Thankfully, none of this noise seems to be audible through my 'phones.

You may be the first to put CTH through that form of testing.
The "fancy" CTH from my sig was built w/same motivation as yours - making it as quiet as possible. And, like your observations, I hear no noise through any of my cans. Pretty much a "black" background, at least for an amp w/a tube in it.

Is your amp cased up? If not you could try to localize/reduce the 120hz & its harmonics you are seeing.

I'd not suspect the heater circuit, as it operates @ 150khz, but you could eliminate it by disabling the switcher circuit & using 6v or 12v battery to power your tube's heater. And doing measurements as well as listening tests in the comparison. IIRC, w/new C3H value & target tubes I could not discern a diff. If running tubes w/higher heater draw than we spec, raising C3H to 470uf might yield better measurements.

You could also play around w/physical shielding (e.g. sheet metal boxing of PS sections) but, for likely the same reason you are seeing this noise, there is little room for this w/160+ parts in a 3" x 5" space. E.g. the higher noise in Right channel makes sense looking @ the proximity of the right channel parts (ICR, etc) to the PS section.

In the end you may have to enjoy the CTH for what it is

 

[runeight]

Quote:

Originally Posted by helzerr /img/forum/go_quote.gif Is this CTH (equipped with 12BH7A) noise floor as measured by RMAA typical? While I understand voltage doubler supplies like the one used for CTH's B+ may produce this sort of noise; with only ~2 mA load and capacitance multipliers I wouldn't expect this much 120 Hz and harmonics to be present at the output. Perhaps the heater supply is responsible? Thankfully, none of this noise seems to be audible through my 'phones. Note, this amp is built with R18=100Ω and the 330 μF C3H update installed. I tried another 12BH7A with nearly identical results. RMAA testing the DAC (y1) and another amplifier (Mini3) yield virtually no noise... In my quest to build the quietest CTH on the block, the case is grounded to the SG point and all internal signal wiring is shielded, with each shield connected to its respective signal ground at only one end of the cable to prevent ground loops - I.E. input wiring shields are tied to IG at the volume potentiometer, output wiring shields tied to OG at the output jack, and power wiring is twisted-pair...

The noise spectrum seems a bit high, but not totally unexpected. An amp like the mini3 uses a DC walwart plus IC regs to generate low voltage supplies. It is pretty easy in this case to make very clean supplies for several reasons:

1. The actual rectifiers (and their noise) is in the walwart at the power outlet
2. The IC regs are pretty good at ripple rejection
3. The low voltage (16V) makes it possible to use very large electrolytics in the PS

In the case of the CTH, in order to get the high plate voltage we need to bring AC into the enclosure for the doubler. This places all of the diode rectifiers at the back of the box instead of 6 feet away at the wall. These diodes do radiate into the rest of the circuitry. Furthermore the HV in the doubler requires HV capacitors. Hence they must be smaller in order to fit them. Finally, there is no room for IC regs and while the cap multipliers are good they are not truly active regulators and don't have as good ripple rejection.

So while any and all amp comparisons are fair in my book, this particular comparison compares very different beasts.

Given the original design parameters on the CTH - make a very compact tube amp where you can get 80V on the plates of the tubes from a 24V walwart along with the LV and heaters supplies some design tradeoffs had to made. The PS tradeoff is that the noise is not zero, but below audibility under normal circumstances.

Edit: What I meant to say at the beginning is that the left channel seems about right but the right channel seems too high.

 

[helzerr]

Quote:

Originally Posted by cfcubed /img/forum/go_quote.gif In the end you may have to enjoy the CTH for what it is

Yes, it's cased up and (following the advice from runeight's sig) I don't intend to modify it at this point - my concern is more along the lines of "is this normal, perhaps I botched something during the build that's leading to this noise"...

Quote:

Originally Posted by runeight /img/forum/go_quote.gif So while any and all amp comparisons are fair in my book, this particular comparison compares very different beasts.

It wasn't my intention to compare CTH and Mini3; I was merely attempting to show my RMAA methodology was immune to noise. Regardless of the RMAA results, both amplifiers pass the comparison that _really_ counts - they are a joy to listen to!

Quote:

Originally Posted by runeight /img/forum/go_quote.gif Given the original design parameters on the CTH - make a very compact tube amp where you can get 80V on the plates of the tubes from a 24V walwart along with the LV and heaters supplies some design tradeoffs had to made. The PS tradeoff is that the noise is not zero, but below audibility under normal circumstances.

Since this is my first foray into tube audio, my expectations may be a little too high

. Do you think a slightly larger / lower ESR C10P would help; perhaps there's a way to squeeze one onto the board? The BOM suggests UVR2C101MHD for C10P; might UCS2C101MHD, ECA2CM101, or 16x20mm (should fit in BOM case?) 150μF UCS2C151MHD or EKXG161ELL151ML20S be a superior substitute for this capacitor?

I do not fault the design - I think it's amazing how much performance and versatility has been literally squeezed into this amplifier!

 

[helzerr]

I believe my testing method may be flawed... After much critical listening, the noise simply isn't there with headphones connected to the amplifier, even via highly sensitive (110 dB/mW) AH-C551 IEMs.

By running the CTH output into a very high-Z load such as the line-in on the measurement sound card, the test circumstances do not mimic the amplifier feeding a normal headphone load.

I need to build a load resistance which I can tap the measurement off of, to perform a more realistic test.

Edit: I plan to build something similar to this headphone dummy load - http://tangentsoft.net/audio/hp-dummy.html

 

[aphexii]

Hey everybody! So i accidentally posted this in the tweak thread a week or two ago, but now i've come down to actually ordering and some of the parts have come back in stock, while others haven't (some of which are months backordered, others which have been discontinued)

If anyone could help me figure out suitable replacements for the following, it would be greatly appreciated!!


Mouser #:647-UVP1E100MDD
Mfr. #:UVP1E100MDD
Manufacturer: Nichicon
Desc.:Aluminum Electrolytic Capacitors - Leaded 25volts 10uF 5x11

Mouser #:581-BQ074E0224J
Mfr. #:BQ074E0224J--
Manufacturer: AVX
Desc.olyester Film Capacitors 100V 0.22uF 5% Lead Free

Mouser #:511-TL082CN
Mfr. #:TL082CN
Manufacturer: STMicroelectronics
Desc.:Op Amps Dual Gen Purp JFET

Mouser #:140-XRL100V100-RC - Mouser suggests 140-REA101M2ABK1020P as a possible replacement
Mfr. #:140-XRL100V100-RC
Manufacturer: Xicon
Desc.:Aluminum Electrolytic Capacitors - Leaded 100V 100uF 20%

Mouser #:505-MKS4.1/100/10
Mfr. #:MKS4-.1/100/10
Manufacturer: WIMA
Desc.olyester Film Capacitors 100V .1uF 10%

Mouser #:647-UPW1H331MPD
Mfr. #:UPW1H331MPD
Manufacturer: Nichicon
Desc.:Aluminum Electrolytic Capacitors - Leaded 50volts 330uF 10x20 20% 5LS

Mouser #:581-BQ014D0222JDD
Mfr. #:BQ014D0222JDD
Manufacturer: AVX
Desc.olyester Film Capacitors 2200PF 35V 5%

Mouser #:512-MPSA42
Mfr. #:MPSA42
Manufacturer: Fairchild Semiconductor
Desc.:Bipolar Transistors NPN Transistor High Voltage

Mouser #:512-MPSA14
Mfr. #:MPSA14
Manufacturer: Fairchild Semiconductor
Desc.arlington Transistors NPN Transistor Darlington

Mouser #:505-MKS21.0/63/5
Mfr. #:MKS2-1.0/63/5
Manufacturer: WIMA
Desc.olyester Film Capacitors 63V 1uF 5%

Mouser #:647-UVP1C101MPD
Mfr. #:UVP1C101MPD
Manufacturer: Nichicon
Desc.:Aluminum Electrolytic Capacitors - Leaded 16volts 100uF Snap-In Audio

Mouser #:551-EC2-12NJ
Mfr. #:EC2-12NJ
Manufacturer: NEC
Desc.:Low Signal Relays - PCB 12VDC NON-LATCH

 

[aphexii]

^ bump for assistance!

 

[cfcubed]

aphexii - This sort of thing happens in DIY - you have to work to find replacements for spec'd parts. Since CTH has so many parts, there's more opportunity for some be to discontinued or back-ordered.

Here's ways to reduce your list, possibly to just the relay:

1. Use Mouser's "Show Similar" to see possible subs. Compare specs (value, rating, size, lead spacing). Values & ratings (=> spec'd rating) are important, size & lead-spacing less so. As I said elsewhere, there are pics of the completed PCB to view that may provide size guidance (some parts can be a bit larger).
2. If show similar doesn't, use Mouser's great searching facilities to try to ID parts w/same attributes.
3. Since some parts come from Digikey, perhaps look for remainder there?
4. If you've ordered from Mouser in the past you should have their HUGE catalog by now, using it is sometimes easier than their search.

Sometime you just need to put in the time. I've spent countless hours searching & locating parts/replacement parts using Mouser's & Digikey's searching function. My guess is you could get this list down to maybe a couple tough ones. E.g. I think you can sub common dual opamps (e.g. OPA2134) for the one spec'd, same for the singles.

 

[helzerr]

I have tested a 150uF 160V replacement for C10P, which fits comfortably in the BoM case:
 
Mouser # 647-UCS2C151MHD, 16 mm Dia. x 20 mm L, 7.5 mm lead spacing.
 
This appears to be a worthy upgrade to the power supply components, as RMAA testing yields lower noise measurements in the 60Hz region. Probably not significant enough to bother replacing an existing C10P, but worth consideration for inclusion into any new builds.

 

[cfcubed]

That's good research & info helzerr.  I wonder if your tests would show improvement for increasing C3H beyond the BoM 330uf (low impedance).
My testing, posted here, showed no noticeable improvement going beyond that at least for target tube heater loads.  But perhaps a 470uf 50v low impedance cap could fit there & show some measurable improvement in your tests.  With CTH's size goals more sophisticated filtering could not be pursued.
 
And if you are up for it you could try increasing the values of the cap multipliers (c12p, c11p, etc) to see if that yields any results.  Not sure any of this will tho.
 
aphexii - Wonder how its going w/your CTH BoM replacements.

 

[helzerr]

I may be able to locate and test replacements for C5P, C11P, & C12P.  The others are hot-glued in place as I consider the build finalized and am thrilled with its performance as-is.  Perhaps if I build another...
 
647-UPW2A330MPD6 (33uF 8x15mm low impedance) or 647-UPW2A470MPD6 (47uF 8x20mm low impedance) look like good candidates for C11P & C12P, but 47uF may be too high a value (B+ rise may take longer than the ε12 turn-on delay with too much capacitance here unless R2P & R3P values are also lowered.)
 
I'd be tempted to try a UPA1V561MPD (caution, part only rated 35V, application calls for 50V) for C3H if I could get my hands on one...

 

[steven2992]

I just finished my CTH and when doing the setup everything checks out okay exept the voltage across pin 1 and 6 on the tube. I get -0.6 with the red probe on pin 1. I tried reflowing components but hasn't helped. I'm a bit lost here because I was very carefull with installing the right parts. 

 

[fishski13]

Quote:

I just finished my CTH and when doing the setup everything checks out okay exept the voltage across pin 1 and 6 on the tube. I get -0.6 with the red probe on pin 1. I tried reflowing components but hasn't helped. I'm a bit lost here because I was very carefull with installing the right parts. 

measure pin1 to ground and then measure pin6 to ground.  pin1 is the plate of 1/2 of the dual triode and pin6 is the plate of the other 1/2 of the triode.

 

[steven2992]

pin1 to sg measures 96.6v and pin6 to sg measures 97v