2. amps-preamps
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Atma-Sphere MP1 with single-ended inputs and outputs? Adapters OK?

I had the opportunity to hear an end-game system (for me) that was fronted by an Atma-Sphere MP1 preamp and their Novacrons.  I was blown away by the overall sound, and am highly considering starting purchases toward that system for myself, but had the following questions I'm hoping someone could answer.

In short, the purchase path for me is likely going to last 18-36 months, given the costs of the various components and speakers, so I'm going to have to be living with "hybrid" systems for that time period while I spread the purchases out.

My current weakest link is my pre-amp (although it's not weak, but...).  If I start with the MP1, both my source material (VPI Classic turntable) and amps (Thoress 845 SET monos) are single-ended.  I'm sure I could get an appropriate balanced output for the VPI (I think?) and my DAC already has balanced out (which is my only other source), but what loss in sound quality will I experience adapting for singled ended RCA connections out from the MP-1 to my SET monos?  My current set-up has short (1.5M) runs, so the length of cable won't be an issue for me in single-ended.  But will using something like adapters or the tape outs on the MP1 negate any sonic benefits of the MP1 I might otherwise be getting for the time period I'll be using single-ended amps?  BTW, I LOVE the Thoress 845 amps, so they will likely be the last part of the chain I will consider replacing, and I may choose to keep them in the end-game system depending on the synergy.

Thoughts or experience with the MP1 and single-ended output?
parsons

Showing 7 responses by atmasphere

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atmasphere
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@parsons Just a FWIW, any SET can be set up (if you see what I did there 😁) to use both phases of a balanced signal. Its easy to do, inexpensive and does not change the character of the amp. By doing it this way there is no buzz or the like. That is how I have used the MP-1 (or MP-3) with SETs in the past and it works a treat.

The hardest part of this mod is finding a spot on the amplifier chassis to install the XLR connector. The rest of it does not require any component changes in the amp. Normally the SET accepts a signal on the grid of its input tube; but it can accept a signal on the cathode of that tube as well. So we just make that input available on the XLR connector.

atmasphere's avatar
atmasphere
12,088 posts
 

I think the isolation transformer should have gain volume control and let you get the right gain after the transformer, as the signal will become weak  if no gain increase and that's the reason I give up the way to use Jensen isolation.

There are a variety of different Jensen transformers. Some have step-up, others step down and still others are 1:1, doing only isolation or conversion from balanced to single-ended (or vice-versa).

atmasphere's avatar
atmasphere
12,088 posts
 
@luisma31  Tie the shield at both ends, or only at one end. If only at one end traditionally that would be at the preamp end, but you'll find its not of much concern. What is important though is that pins 1 and 3 are tied together, and that connection is then tied to the RCA ground at the other end of the cable, and that there is a shield as well.
atmasphere's avatar
atmasphere
12,088 posts
 
Sure, but just so you know Cardas and many others make adapters that will do the job. Just make sure no matter who you order from that pin 1 and pin 3 must be tied together.
atmasphere's avatar
atmasphere
12,088 posts
 
@dbphd , yes- an OTL is a tube amplifier without an output transformer. In our case, the output of the amp is direct-coupled (so our our preamps).
atmasphere's avatar
atmasphere
12,088 posts
 
@dbphd  that sounds like that should work.

The Jensen Isomax transformer works very well converting balanced to SE. But, correct me if I’m wrong @atmasphere , doesn’t connecting a stand alone transformer inline w/ an OTL affect the sonics?
Of course, and OTL has nothing to do with it. I prefer to use adapters myself (the ones we make have the correct connections).

Now another way of dealing with this when using an amplifier which has a single-ended input (in particular tube amps) is to modify the amp so that it can process the inverting signal from the preamp (in addition to the non-inverting signal). This is a lot easier than it sounds and the character of the amp is not changed at all. All amplifiers have the ability to do this but the 2nd input (the cathode of the input tube) is usually just grounded.

Essentially you make a connection from the cathode to pin 3 of the XLR input connector via a coupling cap. If the amp is equipped with a cathode bypass capacitor on the input tube that is the cap that is used, if not, you add one. The hardest part is mounting the XLR input connector on the chassis. If any of this sounds even slightly mysterious have a technician do it for you. The cathode input connection is very low impedance, but our preamps have no problem driving that. I've been using this type of connection for years on several different amplifiers.
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atmasphere
12,088 posts
 
The Atma-Sphere MP-1 and MP-3 are both available with optional RCA outputs (in addition to the dual balanced outputs). When this option is added there is also a switch installed to switch between single-ended and balanced operation. This is needed as otherwise you can encounter a buzz. When the switch is in the single-ended position the output of the preamp is then single-ended regardless of the connector used.

You can also use and adapter or cable that does the conversion. If so, it is important that pin 1 and pin 3 of the XLR be somehow connected together. This will not harm the preamp in any way and is needed to prevent buzz.

The reason the buzz can occur is that the output of the preamp (pins 2 and 3 of the XLR) is a true balanced circuit, and therefore is 'floating' with respect to ground. If only one side of the output (for example pin 2 of the XLR connection) is used, it will behave like an open input, because that is exactly what it is. To solve this, one side (usually pin 3) is tied to ground (pin 1). 
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