2. amps-preamps
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Need advice from OTL experts

I have recently purchased a used pair Transcendent T8 OTL mono blocks (80 watts into 8 ohms) for my Quad 63 ESL speakers. For many years before this purchase, I have used ARC Classic 30, Classic 60, D115 and ASL Explorer 805 SET mono blocks with the Quad 63. I am a strictly vinyl person and my preamp is the ARC SP8 with a VDH Frog HO MC cartridge on a Oracle premier II TT. My speaker cable is MIT MH 750 extended, VPI phono cable and Cardas Decade interconnect from Pream to amps. The bottom line is I have never heard better sound and music coming out of these Quads before the OTL's and I am very pleased with this amp and speaker combination. The sound very pure, clean and precise without the typical tube coloration and faster than all the amps I had before. The gentleman whom I bought the OTL's from told me my SP8 preamp's output impedance at 1000 ohms is too high for these amps and speakers. I have owned this SP8 for many years because I love its phono section and I hate to part with it. My question is if this is a true statement with science behind it and if I would benefit from switching to a preamp with lower output impedance. I have looked at Transcendent web site. Their preamp has output impedance of 200 ohms and some other preamp brands have as high as 2000 ohms. Many thanks in advance.
blptwp
12 responses Add your response
twl
3,251 posts
 
It depends upon what the input impedance of the amp is. I looked at the Transcendent website, but they don't list the input impedance. The 1000 ohms output impedance of your preamp is a little high, but not really out of the ordinary. Many tube amps have a pretty high input impedance like around 100k ohm. This would work fine with a 1k ohm output impedance of the preamp.

OTL's work great on Quads because of that speaker's impedance values, and even do better than many transformer amps on them, because of the speaker's impedance rise to about 30 ohms in the bass regions. This helps out the OTL weakness which is high output impedance, so in the bass regions the Quads help out the damping factor by rising in impedance to 30 ohms. A pretty good thing for OTL's.

I would say that you have a pretty nice sounding system.

You will have to check with Transcendent about the input impedance of the T8, and their recommendations on the output impedance of the preamp.
blptwp OP
40 posts
 
Twl,
Thanks for your input. You are right, the the input impedance of the T8 is around 100K ohm.
blptwp
sean
6,231 posts
 
Given the figures that you've listed, i would not worry about impedances in the least. According to theory, the amp is in no way loading down the preamp. It should sound like "magic" and according to what you've stated, it is working as well or better than you expected. Enjoy and forget about the previous owners' comments. Sean
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cdc
1,783 posts
 
What's "OTL" mean? Over the litterbox? Out to lunch?
thomasheisig
568 posts
 
OTL : Over the Loosers
twl
3,251 posts
 
Cdc, OTL is an acronym for "Output Transformer-Less". It refers only to tube amps, because SS amps don't need an output transformer. The output transformer's role is to impedance-couple the inherently high output impedance of a tube, to the relatively low impedance load of a speaker. If the impedance of a speaker is below the output impedance of the amp, the amp will typically become unstable. OTL amps eliminate the transformer by running a number of output tubes in parallel, thus lowering the output impedance of the amp. This eliminates the output transformers, and any of the negative effects that the transformer may have on the sonic quality, especially at the frequency extremes. But, the additional output tubes in parallel make some problems of their own, notably the fact that they usually end up with a higher output impedance than their transformer-coupled counterparts. This makes them better performers with higher impedance loads like 16 ohm speakers, because of the damping factor.
fdriver
17 posts
 
I've lived with the Atma-Sphere OTL amps - outstanding products and high powered. I also had good experience with a Futterman-type OTL, called the Prodigy M1. I used both tube and ss preamps, and never had an issue.

FWIW: Not everyone thinks that transformers are a bad thing. McIntosh actually uses a less sophisticated version (AutoFormer) for their SS gear!!
brucegel
151 posts
 
Could it be that many manufacturers lack the cojones to market OTL'S because it is a niche with fewer wallets...hmm.
sean
6,231 posts
 
Brucegel: I think that OTL's are a LOT harder to get to work well than a standard tube amp with output transformers. The transformers lend a WIDE margin of "buffer zone" to a tube amp. Without that buffer zone, the linearity and overall performance of the circuit is forced to bare its' soul into whatever load you throw at it. Most designers / manufacturers aren't willing to risk their reputations by taking the chances that OTL designs and the various loudspeaker loads force upon them.

Like anything else, if you are good at what you do, regardless of what it is and what it costs, you will have people to market your product too. OTL designs are no different than any other part of the market, even though it may be a smaller segment overall. Sean
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brucegel
151 posts
 
point well taken sean...my admiration for the succesful otl designers is rather obvious.
cdc
1,783 posts
 
Great explanation TWL. Thanks.
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