Most 300b amplifiers are single-ended.
Most designers think that there is no point to adding a balanced input to such an amplifier.
But the fact is there is a way to do it (without an input transformer too) that allows the amplifier to process both phases of the balanced signal.
Any single-ended amplifier can be easily modified to do this without changing the character of the amplifier. FWIW, most single-ended designers don't know this is the case.
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Most designers think that there is no point to adding a balanced input to such an amplifier.
What would be the point? |
What would be the point? Depending on the design of the specific preamp that is being used, perhaps less sensitivity to cable effects, less susceptibility to ground loop issues, and in some cases perhaps better sound from the preamp’s balanced outputs than from its unbalanced outputs. There seems to be a belief among some audiophiles that balanced interfaces can only be beneficial if the internal signal paths of each of the components are balanced. I see no basis for the belief. Depending on the specific designs a balanced interface may provide better results or worse results than an unbalanced interface, even if some or all of the components are not "fully balanced." Regards, -- Al |
Thanks Al and Ralph
I have a 5 meter run of Audience SX balanced interconnect between my VAC Sig IIa preamp and my Clayton M300s and I'm thinking about a Mastersound 300B amplifier to use with some monitors. I do not have a quality 5 meter run of unbalanced interconnect. May I use a female XLR to male RCA adapter at the unbalanced amplifier input? If not I'll just purchase a 5 meter RCA run or let Ralph custom modify the input to balanced.😊 |
Audiobrian, your question gets into the fact that most XLR-to-RCA adapters connect the inverted signal on pin 3 to ground (pin 1). That can be appropriate for many preamps that have transformer coupled outputs (as yours does), but would not be suitable in many cases where both the inverted and non-inverted outputs are directly driven from active devices. (Although for those situations I believe Cardas can supply adapters on special order that leave pin 3 unconnected). I note the following statement in the manual for your preamp: Main outputs are available as unbalanced via RCA jacks or fully balanced via XLR jacks. You may use either set, but not both simultaneous/y. The GROUND switch must be set correctly for the output configuration being used. Incorrect setting will result in either excessive buzz, or cause gross distortion of frequency response and output. For power amplifiers with RCA input jacks, you will normally use the "RCA GND" setting. You may also choose "XLR/RCA UNGND" if it is necessary to break a ground loop. For power amplifiers with XLR input jacks, you must determine if the input is truly balanced, or if either pin 2 or 3 is actually tied to ground (unbalanced XLR). For true balanced inputs you may choose between "XLR GND" and "XLR/RCA UNGND" for lowest hum level.
For power amplifiers with unbalanced XLR inputs, select "XLR/RCA UNGND". If the amplifier ties pin 3 to ground, you may also use "RCA GND". Do NOT use "XLR GND".
This leaves me somewhat confused, because in looking at photos of the front and rear panels of the preamp, including the photos in the manual, I don’t see a "Ground" switch anywhere. But if there is such a switch, based on these statements and on the fact that the outputs are transformer coupled I suspect it would be suitable for you to use typical XLR-to-RCA adapters that ground pin 3, but I suspect you would want to set the Ground switch per the last paragraph, as if you were connecting the preamp to an amp having unbalanced XLR inputs. It would probably be best, though, to give Kevin Hayes a call at VAC, and present the question to him. Good luck. Regards, -- Al |
Cost. I know some professional amplifiers that have balanced inputs, but internally short pin 3 to ground. It's kind of a waste, but it's a necessary convenience feature there.
In the residential world, you could add an XLR input, and do the same thing, but you won't get any benefit from it unless you actually have a balanced input stage, and even then, whether it's a benefit or not would be debatable. It would involve more circuit complexity.
By leaving the XLR connector out they can save money, keep the price a little lower, and leave it up to you if you must have XLR. I know of very few preamps that don't offer RCA outputs, so most of the time this makes sense.
If you need XLR to minimize noise due a longer run, you'll probably want to use a transformer at the amp end, but only very long runs prone to noise, or near powerful sources of noise such as a radio tower would benefit.
Best,
Erik
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To audiobrian's original question ...
I'd hazard a guess that in the population of SET owners, less than 1% have balanced preamps. I don't have industry numbers on this. Anecdotally, I know of only one SET owner who's running a balanced preamp.
There comes a point when making design decisions that you don't design for such small numbers.
Cheers,
Thom @ Galibier Design
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Ralph,
Are you using diodes to separate signal from the single output tube per channel? |
May I use a female XLR to male RCA adapter at the unbalanced amplifier input? No. That is the same as single-ended. In the residential world, you could add an XLR input, and do the same
thing, but you won't get any benefit from it unless you actually have a
balanced input stage, and even then, whether it's a benefit or not would
be debatable. It would involve more circuit complexity. With the input I described earlier, this is essentially not true. While no SET has a balanced input, they **all** have differential inputs, else the amplifier would be unable to amplify. Try shorting the input grid to the cathode of the input tube and you will see what I mean. The tube amplifies because of what is different between the grid and cathode. |
No. That is the same as single-ended.
Ralph, yes, it would amount to a single-ended connection. But he wants to use his existing 5 meter balanced cable to connect to the single-ended input of the amp he is considering. It seems to me, given the info I quoted from the manual for his preamp (and aside from the uncertainty I stated about the existence of the ground switch the manual refers to), that he should be able to use that cable with an adapter. Best regards, -- Al
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^^ He can, but 5 meters is a long ways to go single-ended, even if the cable is balanced. The adapter, if properly wired, will tie pin 3 to ground. At that point the cable capacitance will increase.
But with a simple mod (which is done by adding a coupling capacitor into the input tube's cathode circuit, which is tied to pin 3 of the XLR, pin 2 being tied to the RCA signal connection) then the balanced preamp can drive the amp directly with no need for an adapter.
The amp at that point will be operating with a differential input. It won't be balanced, but it will work better than with single-ended.
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Depends on the adapter. ;) Putting an active stage or transformer at the amp end will do a pretty good job. However, are yo sure there's something that needs fixing? 5m might be fine with shielded, single ended cables. Maybe. Worth trying first.
If you need transformer coupling, look up Jensen transformers, they have some great devices to do this for you.
Best,
Erik
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A truly balanced circuit is like having 2 completely different circuits ....one for the + side, and one for the - side. This is a very expensive piece to build. Tube circuits are expensive to begin with....this would about double the cost. Designers aren’t sure the upgrade is worth the cost on the battlefield of sales.
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I share the same thought stringreen,
Those low-powered 300b single tube per channel amps can’t by any means be fully balanced. The can however have as previously mentioned a differential input stage. |
