KSTR said:
These currents are much more significant than they might first seem to be. These currents end up going through all the grounds in the system. On the low level stuff they can subtly modulate the audio. And the "HF chop" (actually the charging current for the filters) can have a broad spectrum, and the magnetic field can do the same. I currently float supplies and derive ground at the regulator to keep it clean.
john curl said:
Didn't the regulators compensate for the DC shift? Or was this a dynamic shift of some sort?
The only reason I asked is that I have trouble getting a solid "minimal hum" noise floor with the dual bridge approach and I'm curious if there is something I'm missing and this is not an issue.
Mike.
MikeBettinger said:
I thought about your comment, as usual I'm in another universe.
I would just trim one of the regulators to remove the offset. I experience difficulty (relative to a single bridge an a solid reference to the centertap) with the dual bridge and virtual ground.
As Gilda would say, nevermind.
Mike.
1audio said:
Hi Demian,
Apologies to John for a slight OT excursion, but I have only just seen this, and these comments do relate to HQ preamp circuit design choices.
My DMC-10 was fitted from new with the 10k Alps Blue (or is it Black-Beauty?) pot you mentioned and (Stackpole) switched discrete resistors for the balance control. After returning it to Spectral following a minor fault, when I also agreed with Rick Fryer to having the latest 'Delta Revision' update carried out, it was returned with a Noble pot, instead, and a 'detented' 100k Noble rotary pot for balance purposes.
Unfortunately, I was unable to assess precisely what this did, in isolation, for the sonic results. Several other significant mods were carried at the same time including the addition of cascoded dual J-Fets (on piggy-back daughter-boards) in the front end, together with some changes to the HF stabilising arrangements, to mention a couple.
Shortly afterwards, I changed just the Noble volume pot to a (wickedly costly) hand-selected conductive-plastic Penny & Giles pot, and like you, I still found this to be 'wanting' in performance, although it was an improvement on the Noble pot it replaced.
I was only finally satisfied when I replaced the entire volume/balance arrangements with 2 Shallco switches populated with all Vishay bulk-foils. This meant operating 2 controls to set the volume, although being 'stepped' - accurate L/R balance was still easy, but the improvements in sonic performance all around were so so marked that this was a very minor inconvenience.
This would have been over 20 yrs ago IIRC, but since then I would never use any proprietary rotary pots in any of my designs, although for a recent commercial commission I did adopt relays, purely for 'enforced' remote control purposes.
Here I concur with Sigurd, and have found that the TX2352 (naked Vishay resistors) from Texas Components are truly excellent, even when switched through relays (although Shallco 'manual' switching is sonically superior!) and, of course, they can be made in tight tolerances to ANY value to 6 decimal places, which provides absolute accuracy to fractions of a dB of attenuation.
Also, to reinforce John's earlier comments on the audibility of ferrite beads (which I will not use anywhere nowadays following many years of careful listening trials - although initially I thought that the potential problem was restricted merely to high-current locations), I also removed the ferrite beads in the phono inputs at one stage, and there was a clear sonic benefit subjectively. IIRC, these were also added during the update revision rather than being present from when the DMC-10 was new, but they were detrimental to the sonic results, as I duly discovered.
Incidentally, what was the true purpose of the 400R resistors located directly between the inputs and outputs of the regulators on the DMC-10 PCB? Intuitively, I never came to terms with their inclusion, although clearly you must have a had a good reason for this addition which is in effect a resistive 'bypass' to the regulators, themselves.
Regards,
KSTR said:
I have been using discrete bridges since the early
90's build up from fast but soft recovery diodes. At the time, XYIS (sp?) had some that were relatively expensive. In my last amp I used Philips BYV32E-200, which from the spec sheet seem a huge overkill, but they are less than a buck each. They are actually two diodes in a plasic TO3 but I just parallel them on the board.
They are so 'soft' recovery that you can skip the snubber/cap across them which of course eliminates another path from mains to your circuit.
Jan Didden
I too used s Shallco and Vishay combination and agree completely with your conclusion. In mine I ganged left and right volume and simply removed the balance adjustment, its 5 years later and I don't miss it.Bobken said:
While experimenting with things of this sort, I was much surprised by how the sound was negatively affected by a standard ceramic multi-pole selector switch (input selection and tape monitor for my homemade pre). Those Shallco's are expensive but if you want the last word on clean sound they are all but mandatory.
Mercury whetted reed relays were a known good audio switch, anyone care to report on an alternate relay contact scheme?
john curl said:
But this approach is creating a virtual ground to which the signal and the supplies are referenced to.
If the transformer regulation is reasonable and the supplies are accurate and stable relative to the virtual ground, why would the DC drift matter? In an A/B poweramp, where the loading on the supply is a bit more dynamic, I can guess that ground reference drifting might be significant. I'm not sure there would be an issue in a Class-A preamp.
Why did you implement this in the Vendetta? Was there technical reason or purely a sonic benefit?
Mike.
Mike, IF the + and - DC load are not exactly the same, then the supplies will drift until they are, or the circuit stops working. I make symmetrical circuits, so my loads are the same, others do not always have the same current draw from the + and the - supplies. The transformer center tap is a convenient DC ground return, and is sometimes necessary.
hermanv said:
I'm surprised to read that mercury whetted reed relays were a known good audio switch. Both contact arms are made from a magnetically soft high-permeability iron alloy. When energized, the audio signal is surrounded by a magnetic field. I would think from reading this forum, that this is not a good idea.
The resistivity of mercury is about 60 times higher than silver. The advantages are no "contact" wear and the inert gas hermetically-sealed capsule. Good for long life, but I wouldn't think it is so good for audio signals. Also, if you moved the equipment while in operation you could disturb the tiny mercury puddles and accidentally close all the input sources at once in a preamp.
Hi John,
I am certain that controlling a pair of Shallcos in unison could be done via remote-control, but so far I haven't tried this as the need hasn't arisen and it will not be particularly easy. It is clear to me that a little thought and experimentation might be needed for this.
For brevity before I didn't also mention that subsequently I arranged to combine the actuation of the 2 Shallcos in my DMC-10, simply with using a 'jockey' wheel arrangement between the 2 switches, and I don't have any aversion to operating volume controls 'manually'.
The addition of a 3rd gear wheel seemed necessary in order to overcome the normal 'reversing' of direction of operation (rotation) if 2 rotary switches are connected by simple gear wheels when one will turn clockwise and the other will turn anti-clockwise at the same time, when meshed together.
In my case, I had sufficient room to add a separate gear wheel on a sprung 'swinging arm' which fits between the gear wheels attached to the rears of the Shallcos. Because of the constraints imposed by the positioning of the original DMC-10 volume and balance controls (and I didn't wish to spoil the front panel appearance - so the same fixing-centres were mandatory) I had some space between these switches anyway unless I adopted quite massive gear wheels to fill this gap.
Provided that the intermediate gear wheel is a bit larger in diameter than the gap between the gear wheels located on the switches, this arrangement appears to be fine in practice and the arm spring forces the middle gear wheel into contact with both of the switch-mounted gears.
I can also hitch-up the jockey-wheel arm to change the balance, if needed, and there are plenty of accurately-made plastic gear wheels of all sizes available on the market. Not all 3 gear wheels need have the same diameter/number of 'cogs' provided that they are the same on both of the switches, and that they all mesh together satisfactorily.
One of the trickiest parts to accomplish successfully was tweaking the Shallco detent springs as otherwise they were far too strong in action to permit controlling both switches with (either) one of the rotary knobs without the intermediate gear wheel from 'riding up' and loosing sync. Also, with 2 'standard-spring' switches being rotated simultaneously, quite a bit of effort is required which would probably not suit some users.
It turned out to be a fully usable and satisfactory arrangement, but I did need to wind my own springs in the end for this 'one-off', to get things right.
With no detent springs, the rotational torque needed with Shallcos is not great as it happens, and as the contacts are quite butch and large in diameter, a degree or so 'out' in precise rotation would still be acceptable in my opinion, for satisfactory operation.
I am sure that with some thought and effort one could come up with a stepper-motor arrangement or similar, which is controlled by software and a hand-held remote, but with my present commitments, this is not high on my current list of priorities.
Regards,
I am certain that controlling a pair of Shallcos in unison could be done via remote-control, but so far I haven't tried this as the need hasn't arisen and it will not be particularly easy. It is clear to me that a little thought and experimentation might be needed for this.
For brevity before I didn't also mention that subsequently I arranged to combine the actuation of the 2 Shallcos in my DMC-10, simply with using a 'jockey' wheel arrangement between the 2 switches, and I don't have any aversion to operating volume controls 'manually'.
The addition of a 3rd gear wheel seemed necessary in order to overcome the normal 'reversing' of direction of operation (rotation) if 2 rotary switches are connected by simple gear wheels when one will turn clockwise and the other will turn anti-clockwise at the same time, when meshed together.
In my case, I had sufficient room to add a separate gear wheel on a sprung 'swinging arm' which fits between the gear wheels attached to the rears of the Shallcos. Because of the constraints imposed by the positioning of the original DMC-10 volume and balance controls (and I didn't wish to spoil the front panel appearance - so the same fixing-centres were mandatory) I had some space between these switches anyway unless I adopted quite massive gear wheels to fill this gap.
Provided that the intermediate gear wheel is a bit larger in diameter than the gap between the gear wheels located on the switches, this arrangement appears to be fine in practice and the arm spring forces the middle gear wheel into contact with both of the switch-mounted gears.
I can also hitch-up the jockey-wheel arm to change the balance, if needed, and there are plenty of accurately-made plastic gear wheels of all sizes available on the market. Not all 3 gear wheels need have the same diameter/number of 'cogs' provided that they are the same on both of the switches, and that they all mesh together satisfactorily.
One of the trickiest parts to accomplish successfully was tweaking the Shallco detent springs as otherwise they were far too strong in action to permit controlling both switches with (either) one of the rotary knobs without the intermediate gear wheel from 'riding up' and loosing sync. Also, with 2 'standard-spring' switches being rotated simultaneously, quite a bit of effort is required which would probably not suit some users.
It turned out to be a fully usable and satisfactory arrangement, but I did need to wind my own springs in the end for this 'one-off', to get things right.
With no detent springs, the rotational torque needed with Shallcos is not great as it happens, and as the contacts are quite butch and large in diameter, a degree or so 'out' in precise rotation would still be acceptable in my opinion, for satisfactory operation.
I am sure that with some thought and effort one could come up with a stepper-motor arrangement or similar, which is controlled by software and a hand-held remote, but with my present commitments, this is not high on my current list of priorities.
Regards,
hermanv said:
Hi Hermanv,
I couldn't agree more with your first comments, but having tried some mercury-whetted reed relays a while ago, I was not at all impressed with what I heard. Doubtless this will enrage some posters, but I am in the camp which doesn't care for anything magnetic anywhere near to audio circuits, and accordingly even go so far as only using stainless steel (non-magnetic) or brass etc. screws in any enclosures.
Transformers are the only enforced exception (although I am particular about their construction) but I keep these as far away from everything else as physically possible.
Reed relays require a magnetic field to 'close' them for operation, and their contacts (all those which I have seen, anyway) are flimsy with tiny contact areas, which possibly explains their sonic 'attributes' which I would prefer to avoid.
I have posted before about my 'normal' relay experiences, and as mentioned then, after finally selecting a few relays based on their sonic performances through extensive listening trials in comparison with a straight wire, I also destructively tested them for potential constructional problems.
This can matter a lot, as I have discovered over many years of careful trials, and it doesn't follow that an initially fine-sounding relay will perform at this level for very long.
Shallcos are the finest-sounding switches I have yet discovered, but the UK made Blore Edwards very similary constructed silver-contact switches are about on a par. I see a lot of praise for Elma rotary switches, but regret to say that they are not in the same league, even initially, let alone after some normal usage.
Regards,
- Status
- Not open for further replies.