... humming machinery is fine, amplifiers not so much ...
update 12-July-2024
Recent amps have varied considerably in tube complement, circuit gain and feedback.
On Line review comments may no longer apply to stock amps. Transformers may not be the same. CAVEAT EMPTOR!
That being said, reviews are only valid for the writer in his room on his equipment and his program. YMMV
overview
WARNING: Lethal Voltages!!
- Proceed ENTIRELY AT YOUR OWN RISK -
NOTE: This document refers to the Audio 'ground' reference as 0vA.
Earth Safety on an AC receptacle is connected to Earth Safety ('ground' in the AC panel).
Direct Connection of 0vA to Earth Safety may give rise to so called 'ground loops'.
We recommend isolating amp circuit 0vA and Earth Safety with a Loop Break, detailed below.
Arrival
Two heavy boxes arrived, one with an ominous pimple. The assembled amps were shipped in the kit box, too small for such heavy objects, permitting only woefully inadequate packing. Upon removal, the left amp rattled. The power switch toggle had been forced into the switch, destroying it. The impact force was enough to cross-thread and deform the mounting nuts, making removal particularly difficult.
Interestingly, the 12AU7/5963 requires ½ the heater current of the 12BH7. Tube heater supplies are nominally 6.3vac ± 10%, with ±5% providing longer tube life. At 117v line voltage, with 5963/12AU7 tubes & 2xKT88 heater voltage is 104% of nominal. v1/3 are 106% of nominal. v2/4 heater supplies appears designed for 12BH7 drivers with 2x KT120s. See
ieLogical Rolling for a bit on tube rolling and using KT120 in place of 6550/KT88. If line voltage is anywhere near U.S. standard 120v, a bucking transformer is required with standard [2x 12AU7/5963 & 4x KT88/6550] tube complement. Schematic, mechanical and parts list to build the pictured
Bucking Transformer. Optionally, but not optimally, a variac may be used. Beware of cheap Chinese products recommended by Messrs. Latino & Mottram.
Hummmzzz...
surely you're joking, Mr. Latino?
Assembly Faults
Design Faults
how bad can it be?
Per Mr. Latino, pre-purchase [February 2019]:
“I had someone test the M-125s about 9 years ago. He came up with a -95 dB signal to noise ratio. I do not know how that was 'weighted'. Typically with good input driver tubes, you have to put your ears on the grille cloth to hear any hum/noise with speakers of average efficiency.”
Jigs
Power Transformer Polarity
The drawing at left shows induced current flow with transformers with inverted and correct polarity.
The lower block shows the effect of a correct Single Point 0vA. As built, with the nine point 0vA there is resistance between each gain block. 0vA currents shift the reference. With a true single point 0vA, the relative voltage at each gain stage does not shift.
Heater Wiring
The image shows the Heater Polarity concept for a Push-Pull [PP] amplifier. The inverter stage splits the signal into inverted polarities. If Heater Polarity is inverted, it functions the same as the signal and creates a current in the Output Transformer. If Heater Polarity is the same, no signal is created in the Output Transformer.
NOTE: For pairs of tubes, e.g. four output tubes, heater polarity should be inverted between the tubes. Tubes in mechanically the same location, e.g. v1/v2 on the M-125 should have the same heater polarity.
Heater Over Voltage
Heater Polarity Chart
P is in-phase with line, N is out.
Polarity is best determined before the transformer heater wiring is connected.
The right amp v2/v4 heaters were in-phase when they should be out, contributing to the as built 4db SNR reduction.
It was determined by inspection that Modified polarities produce the most benign noise signature in the M-125s I received. Other Revisions might require other choices.
DC Wiring
Prototype at left looked reasonable. Wiring as built very poorly implemented. AC input tie-wrapped to 0vA and DC. Choke input twisted with output contaminating the output with the pulsed DC input.
Chokes are not counter-placed to cancel fields. Chokes sit directly BELOW the Output transformer. Minimizing field radiation pays off.
Mount the chokes with leads inboard. Wire from the rectifier-out/AuthentiCap
terminal to the rear choke leads. Wire the choke output direct to the AuthentiCap. Link the AuthentiCap to the Supplementary Cap Module [SCM]. Connect the output transformer
RED lead to the SCM. Wire the linked AuthentiCap
&
terminals and the added black lead to the SCM input. Wire the ESL film cap across the input and output of the SCM.
Miscellaneous Wiring
GroundLifts/CheaterPlugs are DANGEROUS, Irresponsible and in some locales, ILLEGAL!!
Single Point Ground
Although the M-125s are MonoBlocks, functionally they become one amp when connected to a common source due to their unbalanced inputs. With the Earth Safety ground connected to 0vA, Ground Loops are almost a certainty.
Rod Elliott has a very good page on Earthing and
Ground Loops in particular. The bottom line is that there should be only one 0vA point. FULL STOP! The Earthed chassis should function as a Faraday cage
only and ne'er the twain shall meet.
Driver Board Mount: Replace the 4/40 steel hardware with #6 [3.5mm] nylon screws and stand offs. If using the AB-Q board, its mounting holes need drilling to fit #6/3.5mm hardware.
McMaster-Carr #6 for isolated 0vA:
• #6-32 x 3/4 Hex SO
92319A658 • #6-32 x 3/8 Pan Head
94735A729
• #6-32 x 5/8 Pan Head
94735A732 • #6-32 x 1/4 Hex SO
92319A610
Driver Board 0vA Loop: Verified M-125 Driver v16 & v19 revision ONLY.
The driver board ground runs around the outside of the board, the very definition of a loop. From inspection, it appears the loop to the right of the vertical trace is for bias and the left for audio. Cut the loop to reduce the effect of the shared bias/0vA. Be certain and careful IF doing this revision!!
Other versions likely have a similar fault, but the cut point may be different.
making it safe
Per the
Loop Breaker schematic on Rod Elliott's page, add a 50A [10x device 5A fuse] diode bridge linking 0vA to chassis/EarthSafety ground. Cap provides RF sink. 10Ω/5W resistor is somewhat arbitrary, with values from 10 to 100 ohms generally regarded as satisfactory.
Amazon:
2x 50A diode bridge
Parts Express:
10R resistor •
100nF capacitor.
Space is tight, so it is necessary to enlarge the right-rear output transformer chassis hole slightly to 11/64 in. [4.5mm]. A #8 x 1 in. bolt is required. Link 0vA at the heater ground buss. Tie the Earth Safety link at the same bolt as the AC input Earth Safety.
things are looking up
Unweighted SNR ≅78db L/R. Still not quite the 95db someone claimed to have measured. The waveform however is a dead giveaway for the half-wave bias. Shame on you, Messrs. Mottram & Latino. A full-wave bias would reduce current pulses by 50% and double the frequency decreasing the noise by 12db w bias LPF mod below.
be careful, your bias is showing
Note that some schematics have incorrect C17 & C18 RefDes.
R39, shown here as a link, should be 1kΩ to improve noise performance.
The correct AB-Q connection point is the C18 end of R39.
Left image is v4 bias as delivered. A classic ½-wave rectification with minimal filtering.
Holger @ Erhard Audio, the U.S.A. agent of the AB-Q AutoBias in 2019, was kind enough to tell me that replacing R39 w 100Ω and moving the bias lead to the C18/R39 junction would allow enough current for the AB-Q board and add a little LP filtering. C17 & C18 were reversed on the schematic in 2019 relative to the PCB.
why AutoBias?
it doesn't get any better than this
Unweighted SNR ≅93db L/R. I have to put an ear ON the grille cloth at the woofer to hear anything. Additionally, notice how little hash is present. Absolutely NOTHING can be heard from the tweeter, a vast improvement!
ok, maybe a little better
regulated bias
ieLogical Revision
Binding Posts
I got fed up with the Binding Posts and mount that would not hold.
Replaced with
Vampire BP-HEX from Parts Connexion.
Mount is milled nylon, but can be 3D printed.
M-125 Mount STL &
M-125 Mount SLDPRT courtesy of JD.
banned!
Jan 4, 2020: created a DTAF topic
'teaching the VTA M-125 to sing'
3 lines:
- my VTA built M-125 amps were noisy
- it is possible to make them almost 40db quieter
- a link to this page
4 images showing the noise as delivered and as quietened
who ARE those guys?
“Ian's amps have the 'autobias boards' that are sold by an independent distributor. Ian requested that they be added to his wired amp. The -95 dB signal to noise measurement for the M-125 was done 10 years before the autobias boards were ever created. When you remove the old bias pots and 'piggyback' a second board on top the driver board and start adding wires here and there to connect the autobias board, I am sure that the amp's residual noise level will increase somewhat.”
the project that wouldn't die
RED plated! now I'm pissed
Snubbers
LM334 Temperature Compensation
final thoughts
We used this formula from p11 in
Aluminum Electrolytic Capacitor Application Guide from Cornell Dubilier and then quartered the calculated 1.6MΩ value to 470kΩ for additional safety margin. 470kΩ resistors dissipate ≈0.15W, so use 1W or larger for safety.
Tube voltages can be fatal. The M-125s have exposed parts with lethal voltages. Tubes are hot enough to cause severe burns. These covers are simple to fabricate.
Metals Depot will cut perforated sheet to size. A metal shop can bend in a few minutes.
PDF of CAD dwg 0.063 material fits between chassis & base and secures nicely. Depending on hole positions, a slight corner trim maybe required to clear chassis front and rear panel overlaps.
like WOW!!!
One of the M-125 peculiarities is the same tube for both input and phase splitter. It's not something often seen. The supplied 5963 low voltage gain of ≈20 contributes to lower than specified gain.
In a tri-amped system, it's a PITA. Something a bit higher gain like a 12AT7 or one with a bit less gain might work. A tube mentioned as a lower gain 12AT7 is the
6829. Like the 5963 it's a
computer tube. Tube parameters and Load Line calculations for existing
final Twist
One thing I never liked about the build is the use of untwisted parallel signal wires.
Being of the Not Broke - Don't Fix mindset, I left it alone.
Knowing of course, one day it would bite me...
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