JoeESP9
ESL's & tubes since 83
I really want to hear a transformer coupled preamp
Find somewhere to listen to one of these.
Shootout Test | Eleven Passive Preamps with Transformers (remusic.it)
Cap Coupled vs Direct Coupled
- Thread starter RoSu_30
- Start date
Find somewhere to listen to one of these.
Shootout Test | Eleven Passive Preamps with Transformers (remusic.it)
mostly that transformers with good bandwidth are expensive, and its much easier to get the same kind of results out of direct or even cap coupling. Transistor ones are considerably smaller and less spendy than tube ones but they're still not as small as even a 1960s capacitor. Direct coupling is the better mousetrap if you can get away with it. No phase shifts and no problems introduced by component degradation.
The other problem, especially when dealing with inverter transformers, is the phase shifts make it difficult to do global feedback. A typical active device will give 0 or 180 for a shift. A transformer does +90 and -90 when used as a splitter, so the phase at the speaker is 90 off from whatever the input is. Can't put that in a global loop and have any kind of useful stability.
peguinpower
Well-Known Member
If you were referring to a post of mine, you are right. I made a mistake but did not want to edit it and be seen as changing my statements. 4
yes, transformer coupled designs does not relate to volume control.
Lavane
Lunatic Member
OTL tube amplifiers need more output tubes to get the same output of an amplifier with OPTs. Maybe it was a chassis size thing? The OTL amplifiers I've seen are larger than amplifiers with transformers putting out around the same wattage.
OTL was more of a later development. Need a bunch of tubes in parallel to make the effective working load lower. If one pair wants 6k, parallel makes it 3k, etc. Stack as needed until you can drive 8 ohms. Usually it will be a bunch of triodes wired as a cathode follower so it doesn't take dozens to get the job done.
The output transistors in my amps are directly connected to the output posts of my amp. I never worry much about it. Cap coupling is something you really only see in single ended topologies where they're needed to block half the rail voltage hanging out on the output, and often you'll still see a protection relay because people these days get squeamish watching their woofers go "chug-a-chug" at power on. Complimentary topologies don't have that problem so wiring the transistors straight to the speakers isn't a problem. I can't speak to other amps, but if I have an output fail, it takes the complimentary part out very, very quickly. None of the First Watt amps have any protection in them and I don't build them with it. In my personal opinion, protection relays are more about idiot proofing amps and bad design and/or quality control.
peguinpower
Well-Known Member
Ive wanted a Futterman amp, but they sell for serious dollars given what they are, and their present condition. I have had Moscode amp and Harvey Rosenberg was a fan of Julius. I did it indirectly. The Moscode has traits that my Pass Labs amp exhibits.
peguinpower
Well-Known Member
Well, that really depends on your skills right? I can vacuum bag and fabricate a carbon fiber part to really tight tolerances. I have no confidence in myself in changing ALL the caps on a power amp. If it doesnt work, I wouldnt know where to begin.
I could learn it, but I'd rather pay someone, and earn the money to pay for it in other ways.
Fair enough point. Mostly I was thinking of the fact that all the parts can be had. No need to seek out some special transformer that hasn't been produced in half a century or more. Power transformers are way easier to source or have made than high quality audio transformers. Finding hen's teeth parts to try and copy some amazing but old design is annoying IMO. Also a big part of why I don't enjoy messing with solid state gear. Ironic as it sounds, parts for transistor amps tend to be more difficult to get than tube stuff. It goes obsolete and out of production about every few years, but tubes that were designed 80 years ago are still available.
peguinpower
Well-Known Member
That is great info. I have had a few false starts in rebuilding vintage gear, but I have always realized that my knowledge of electronics is severely lacking and I could end up hitting multiple brick walls
Whoa! In the pass band? How is that? Does the transformer act as a purely reactive element? Guess I'd better drag one of my orphaned KLH 21 projects onto the bench and check out how their transformer splitter behaves. (Or does this not apply to splitters used in solid state designs?)
I don't understand why transformer behavior, when used as a splitter, would be so different from when used as an output transformer (*). What am I missing?
Thanks, and sorry about potentially driving the thread even further off the rails,
chazix
(*) I'm assuming that output transformers produce either 0 or 180 degree phase shifts in the pass band. I can't say I have really confirmed that for myself, though. It would never occur to me to check! I do notice that in tube gear, output transformers are sometimes part of a more-or-less-global feedback loop, so they must not present any insurmountable stability issues.
RoSu_30
Active Member
I want to hear one. I am fascinated by it, as much as I am fascinated by the sound of different amplifier topologies.
Old HiFi Guy
Well-Known Member
My Nikko Alpha II gives you a choice of direct or cap coupled mode. Personally I have never been able to tell the difference, but that may be just me. All I can say is that it sounds good in either mode.
A fellow AK'er asked this question recently, "what is the difference between cap-coupled and direct-coupled" and I typed up a simple description and added an illustration. It's not perfect and this has already been covered in this thread, but here is a picture to go along with the discussion:
Cap-coupled design, at least when said about receivers or amps, described a large capacitor placed between the final output transistor stage and the speakers. The amp is coupled to the speakers via a capacitor. This does a few things, electrically, but, most importantly, it alters the audio signal coming out of the amp and going to the speakers.
The common, modern alternative to this is direct-coupled amp design... where there is no cap between the final output transistors in an amp and the speakers. These designs are often called "neutral-sounding" because they do not alter the audio signal before it gets to the speakers.
Cap-coupled unintentionally modifies the audio to sound a bit warmer and direct-coupled does not modify the sound.
Tube-based amps and early solid state often used a transformer-coupled design, but that is rare to see now.
Since the caps in the cap-coupled design are between the output transistors and the speakers, then the cap-coupled part is in the amplifier or power amplifier section and not the preamp section.
Typical cap-coupled amp diagram (in this case a HK 330B):
Typical direct-coupled amp diagram (in the case a HK 330C):
The common, modern alternative to this is direct-coupled amp design... where there is no cap between the final output transistors in an amp and the speakers. These designs are often called "neutral-sounding" because they do not alter the audio signal before it gets to the speakers.
Cap-coupled unintentionally modifies the audio to sound a bit warmer and direct-coupled does not modify the sound.
Tube-based amps and early solid state often used a transformer-coupled design, but that is rare to see now.
Since the caps in the cap-coupled design are between the output transistors and the speakers, then the cap-coupled part is in the amplifier or power amplifier section and not the preamp section.
Typical cap-coupled amp diagram (in this case a HK 330B):
Typical direct-coupled amp diagram (in the case a HK 330C):
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Ok, I think I fixed it. Can you see two schematic diagrams now?
I like some cap coupled SS amplifiers if designed right.Similar threads
