I think it will be pretty easy to find a preamp with enough output to run it as is. Especially since I what attracted me to this amp is the simplicity of the circuit.
Now, I'm just trying to decide between Edcor or Hammond iron. It looks like I can get the Hammond a lot faster and slightly cheaper (with free shipping and a discount coupon) but the place currently has just one hammond output transformer in stock.
I'm always tempted by the more exotic iron but that stuff tends to be exponentially more expensive.
I'm always tempted by the more exotic iron but that stuff tends to be exponentially more expensive.
I just completed the 30 Watt Mono Block Amp build. It works but has some issues:
1. reduced headroom
2. high THD
3. frequency response
These issues are probably a result of my inexperience in building. I am hoping that someone on this thread is willing to help me work through these issues before I build the 2nd Mono Block.
1. reduced headroom
2. high THD
3. frequency response
These issues are probably a result of my inexperience in building. I am hoping that someone on this thread is willing to help me work through these issues before I build the 2nd Mono Block.
30 Watt Mono Block issues resolved
I've discovered that the feedback circuit was the cause of all of the issues with this amp. Once corrected, the amp performance is excellent(see attached analyses). I am very happy with amp and can't wait to build the other mono block as soon as parts arrive.
I've discovered that the feedback circuit was the cause of all of the issues with this amp. Once corrected, the amp performance is excellent(see attached analyses). I am very happy with amp and can't wait to build the other mono block as soon as parts arrive.
Attachments
I have this book too. What in particular did you have to change in the feedback loop? Is there a misprint or was it some error in the build?
I don't see any mistakes in the FB wiring. When I get chance, I will use substitution boxes to see if the FB can be optimized. Until then, I will leave it disconnected since the THD is very good without it.
monoblocks
Hi - so any further findings on this build ? - i now have every component as per the book and about to start my building - anything i should know before ??
Chris
Hi - so any further findings on this build ? - i now have every component as per the book and about to start my building - anything i should know before ??
Chris
30 Watt Mono Block issues resolved
Both mono blocks function very well with specified FB circuits installed. I was not able to determine why I initially had issues with FB. The only significant change I made was to install a ground bus wire instead of multiple ground points on solder lugs.
Both mono blocks function very well with specified FB circuits installed. I was not able to determine why I initially had issues with FB. The only significant change I made was to install a ground bus wire instead of multiple ground points on solder lugs.
monoblocks
Hello again - im very grateful for your reply thank you , i have fitted all the tag strips by means of riveting to my 4mm alloy top plate , the face of the top plates are now painted - so as are the rivet heads , in theory all the tag strips are grounded to the top plate but i guess i will still run the ground wire around them albeit slightly modified i believe this will be fine but i will do something with the audio grounds - my speaker posts and audio in rca are fitted to my oak plinth and not the top plate so already isolated from the tag strip grounding , i will put some pics up sometime soon of my work -- im really pleased to know your happy with the circuit
Chris
Hello again - im very grateful for your reply thank you , i have fitted all the tag strips by means of riveting to my 4mm alloy top plate , the face of the top plates are now painted - so as are the rivet heads , in theory all the tag strips are grounded to the top plate but i guess i will still run the ground wire around them albeit slightly modified i believe this will be fine but i will do something with the audio grounds - my speaker posts and audio in rca are fitted to my oak plinth and not the top plate so already isolated from the tag strip grounding , i will put some pics up sometime soon of my work -- im really pleased to know your happy with the circuit
Chris
monoblocks
I just wondered - do you maybe have any pics of the inside of your amps ? - are you running the 12bh7
Chris
I just wondered - do you maybe have any pics of the inside of your amps ? - are you running the 12bh7
Chris
kt66 blocks
Chris
Hi there - im getting close to half way through one block right now - im struggling to find much more info on these - no one really answers much - my main concern could be the preamp to drive with ? i have a superb Arcam c30 fmj right now that might be ok -- i would like to find a tube preamp ideally but has to have tape in/out -- what preamp did you use ?
Chris
There should be a number of tube line stages you can find online. One you might consider is the Eric Barbour 'Brute Force in a Line State's (just google that). It should have enough gain, and will drive headphones as well. It doesn't show tape outs, but they wouldn't be hard to add.
great build. How do they sound.
(unfortunately, I don't know enough to be helpful with questions but nice work)
(unfortunately, I don't know enough to be helpful with questions but nice work)
kt66 blocks
well they have about 10 hrs so far - they sound incredible , fantastic vocal ranges with really great bass and sharp high range - all voltages are spot on and very quite - nothing humming through speakers at all the mains transformers have a very tiny amount of humm - im really over the moon with them -- no problem feeding them either just a crappy workshop preamp just now - so thats my next project !! but as i said before i need help with adding a tape loop in
well they have about 10 hrs so far - they sound incredible , fantastic vocal ranges with really great bass and sharp high range - all voltages are spot on and very quite - nothing humming through speakers at all the mains transformers have a very tiny amount of humm - im really over the moon with them -- no problem feeding them either just a crappy workshop preamp just now - so thats my next project !! but as i said before i need help with adding a tape loop in
Attachments
A couple of things could cause negative feedback problems. If the output transformer secondary were connected in reversed, you would wind up with positive feedback instead of negative. To determine if connected correctly, disconnect the feedback loop at one end. Either listen to or measure speaker output voltage with a tone. Connect the negative feedback loop and the level should drop slightly. If the secondary ground leg is not well grounded, you may get some weird feedback characteristics. A note on grounds: The circuit layout drawing does show all ground points wired together. It is not good to depend on the chassis for a common ground. Besides the possibility of terminal grounds becoming loose or corroded over time, copper is a much better conductor than a metal chassis.