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It's done!
The guided tour:
Back: 2 tubes are the Direct Heated Triodes - 2A3 or 45 plug and playable without any circuit changes.
Middle: left tube is the Super Mu 6AS7 and right is the 5U4 rectifier
The tube sockets are mounted in a subplate which is compliantly mounted to the chassis. Rapping on the chassis produces no audible feedback.
Front: the power transformer. From our 98db speakers we can hear just a little tube rush from the 2A3. No hum.
Front panel: Remote controlled selector then power switch left, 120 step volume control on the right. Selector has 5 inputs (also additional inputs for REC on back panel)
Back panel: 6 prs RCA inputs and 3 prs outputs, +6, 0, -6db (this can be changed internally) for system matching which is absolutely necessary to marry DHT and high efficiency speakers.
Inside the chassis: Will post pics next report but power supply is FCUPS and all film cap (big L small c). Ultra Low noise Filament Supplies on the 2A3/45. Auto bias Supply controls the DHT. Etc Etc.
On the bench we get extremely linear and low distortion test results.
OK more later.
DIY - Done Right!
Follow Ups:
Can you post a picture of the back panel?
How would you route the signal to a subwoofer crossover/amp, using the Tram2?
Thanks.
There are 3 pairs of outputs. Different levels. If the subamp doesn't have enough gain adjustment you could use an RCA 'T' connector of one pair.
Net load is the subamp input impedance paralleled with amp impedance.
How about driving the subamp from the amp speaker out through a dropping resistor into the low level (ie RCA) inputs? Then impedance becomes a non-issue and the subwoof sees the same signal as the speakers.
Brian
DIY - Done Right!
Thanks for the response.
Excuse my ignorance, but are the different output levels of the TRAM2 intended to put the volume controlled loudness in a desired range for the given amp/speaker combination?
If I tap off the full range amp speaker outs for the sub woofer inputs, does this, in principle, present a frequency spectrum filtered by the characteristic of the full range amp? Does not tapping from the preamp outputs give the flatter, less filtered, original signal?
In the realm of base harmonic periods, is the relative coherence difference for the amp-out tap versus the preamp out tap significant (aurally)?
The sub amp in this case is a NAD 214 bridged to mono, and the SW is a DIY 18 in. Adire Tempest in sealed large cabinet (low pass filter is Marchand circuit).
Thanks.
Frequency response: better than ±0.5dB 20Hz - 20KHz
S/N Ratio: > 93dBA
THD & N: < 0.2%
All measurements referenced to +18dBu (6V), +10dBu (2.45V) and +2dBu (1V) for the respective outputs
All measurements taken with Sovtek 2A3 and vary with tubes employed
DIY - Done Right!
The concept couldn't be more simple: Use direct heated triodes for linestage duty. Put some signal on the grid, heat the cathode, add B+ to the anode and voila! What could be easier? Works for DHT amplifiers, and parts count is low.But put a DHT into a linestage where signal levels are in the mv range and we crash into noise, microphonics, noise, possible impedance issues and more noise.
Noise can come from power transformer, filament supply, B+, poor grounding, bad layout, not to mention proximity to outside sources of noise.
The Tram 2 starts with good layout. We wanted a single chassis solution together with power transformer. So transformer at the front, triodes at the back close to input/output connections. Tube rectifier and 6AS7 closest to the transformer.
B+ is supplied from the ultra low noise film cap power supply module and the cathode supply comes from our filament supply module with added capacitance.Microphonics must be the middle name of the triode family. We already encounter this in amplifier position, but in a linestage -- talk to your triodes and maybe your speaker talks to you! All slight to jarring level vibrations will mess up the clarity of the sound. Everything around us resonates and thus vibrates. The Tram 2 mounts the triodes on the 3mm tube subplate which is in turn mounted to the chassis with compliant isomount standoffs. Knuckle rap on the chassis doesn't transfer so we've done well.
The other factor is system gain. High sensitivity amplifiers and speakers will feedback through the tubes and exacerbate microphonics. So we have provisioned 3 outputs: +6db, -3db and -12db. The user needs to select in system but in general: amplifiers with input sensitivity of 2 to 6v would match +6db outputs; 0.7v - 2v amps: -3db outputs; below 0.7v amps: -12db outputs.Of course this impacts on impedance so it requires finesse. The Tram 2 can comfortably handle amplifier loads down to 2k ohms.
Result: through our in-house Crescendo speaker system, 98db/w (100db/2.83v/1m in room) we get no hum, just a little tube rush.
We'd originally planned to use a choke to load the anode of the triode but that would push us to a 2 chassis linestage and potentially more noise pickup. So we instead used a 6AS7 together with CCS to load the anode. Result is very low output impedance and very high anode load. The effect is as a super mu follower.
Why DHT OTL linestage?
DHT sound better. On the test bench we get very low noise and very low distortion. What little distortion is there, is of the single ended type which many feel to be consonant with the sound of music.
The Tram 2 as presented can use either 45 or 2a3 types. With some tweeking to the circuit 26, 71, 27 and 56 (non-DHT) can also be used.
One thing to note: as with a formula class racing car, with a DHT linestage we are going for ultimate performance and so give up a few niceties. Care needs to be taken to place the Tram 2 away from noise sources from other gear such as big torroids. Tube selection is important. NOS and ST types generally have less microphonics. We've tried with NOS 45/2A3, TJ meshplate 45/2a3 and carbon/meshplate plate globe types and Valve Art 2A3 single and double plate. Overall the ST types were less noisy. The big globes tend to be more microphonic; ditto for some of the new production types with coil spring tensioners for the grid wire.What you get is very low distortion, linear output with all the tonality offered by direct heated triodes as close to the source as possible.
DIY - Done Right!
Edits: 04/11/10
Hi Brian,
How much tweaking would be required to be able to use a 4v European triode -- say, the RS241 (or maybe an RE134)? In the past I've used combination Yamamoto UX4-B4 sockets to facilitate the physical connection with no switching, but what about the filament and high voltage -- can that be accomodated in this design so it's switchable between 2A3, 45, and (some) 4V DHTs?
Thank you!
Very nice end result Brian. Very nice layout with flexibility to spare. I think your customers will be in for a very nice surprise when they put one of these in their systems. Congrats on getting the noise way down as you did. It is probably one of the most difficult things to tackle with a DHT preamp but necessary to get all of the wonderful detail that they are capable of giving the listener.
The overall look is your usual understated but professional looking piece of gear. I have not seen your listing for the unit but in my experience using silver wire in such a revealing piece of gear is almost a must especially since the runs are pretty short. A good option at the very least. If I did not have one (DHT pre)already I would certainly give this one a try.
I just have one question about the circuit layout. Is the input capacitor there just to prevent possible DC contamination from upstream components? If that is the case then perhaps a note to builders that it may be bypassed if they are sure upstream gear does not have DC. Just one more step in clarity if it is possible.
Again, congrats on a very fine looking unit.
Tony
Thanks Tony, for the encouraging words. We're enjoying the direct flow of music from this one.
regards
Brian
DIY - Done Right!
Hi,
As I had a little itty bitty hand in this, one or two comments.
First, all the signal and ground wiring is silver (soft annaeled) in PTFE Sleeving, 0.2mm for the signal, 1mm for the Ground. The RCA connectors are also silver plated!
The input capacitors are mandatory. In this circuit the DHT operates in fixed bias mode, so cathode straight to ground (okay, via a 12 Ohm bias sense resistor).
In this case the input capacitor (0.047uF PTFE & Tinfoil - only sensible upgrade would be AN Copper or Silver) is essential, without it the volume control would short out the bias and/or send the bias voltage to the source.
The benefit of using fixed bias is that we can dispense with the cathode Resistor and capacitor usually needed. And trust me, a 0.047uF PTFE & Tinfoil Capacitor in front of the grid sounds much less intrusive than a 100uF electrolytic capacitor (or a Nicad Battery) on the cathode... :-)
As it stands the signal goes:
Input ->
Volume Control ->
0.047uF PTFE & Tinfoil ->
DHT Stage ->
10uF Obbigato Copper in parallel with 2.2uF Polypropylene & Tinfoil ->
Output
It is hard to better this without bring in worse problems through the back-door.
Ciao T
Thank you Thorsten. I did not mean for you to give away the store so to speak by posting the schematic (in a sense). Glad to have it cleared up though. As usual, your answers are clear and concise.
The reason I asked though is that I built a 26 preamp using Gary Pimm's schematic which does not use an input cap and is also fixed bias. I agree that fixed bias is the way to go especially in a preamp as the need for amplification is usually very small if needed at all.
Regards,
Tony
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