Author Topic: Mono block project  (Read 2839 times)

Offline Mars

  • Commercial Member
  • Trade Count: (+1)
  • *****
  • Posts: 1,250
  • Total likes: 77
  • “The path is the goal.” Mahatma Ghandi
    • Mars Tube Audio
Mono block project
« on: August 29, 2019, 09:25:04 PM »
Hi All

I have been playing around with ideas for a mono block design for ages. Of all the designs that caught my eye, the EICO HF60 design somehow stayed with me. I drew up some lines and the engineers complied. I took some photos:









OEM manufacture of Tube Audio
Power and Integrated Amplifiers
Phono Pre-amplifiers
DIY Kits
DIY Workshops

Offline fredeb

Re: Mono block project
« Reply #1 on: September 01, 2019, 09:26:46 AM »
That looks absolutely fantastic Karel - beautiful !
...evolution is the gradual development and stratification of progressive series of wholes, stretching from the inorganic beginnings to the highest level of spiritual creation.
Jan Smuts

Offline CAD

  • Trade Count: (+78)
  • AVForums Grandmaster
  • *****
  • Posts: 7,253
  • Total likes: 231
  • Keep Calm,Be Quiet and Listen to the Music
Re: Mono block project
« Reply #2 on: September 01, 2019, 12:33:31 PM »
+1 Looking good so far  :thumbs:

Subscribed and please keep posting pics
Light travels faster than sound. This is why some people appear bright until you hear them speak.

Offline rakumatr

Re: Mono block project
« Reply #3 on: October 01, 2019, 09:29:34 PM »
+1 Looking good so far  :thumbs:

Subscribed and please keep posting pics
Ditto

Sent from my SM-G570F using Tapatalk

"If music be the food of love, play on, Give me excess of it; that surfeiting, The appetite may sicken, and so die."
 - Twelfth Night Act 1, scene 1, 1–3

Offline Mars

  • Commercial Member
  • Trade Count: (+1)
  • *****
  • Posts: 1,250
  • Total likes: 77
  • “The path is the goal.” Mahatma Ghandi
    • Mars Tube Audio
Re: Mono block project
« Reply #4 on: October 17, 2019, 08:32:16 PM »
I've built the one circuit to test the concept of a balanced/unbalanced fixed bias mono block. First I test the amp with a transformer i had on the shelf. After a lot of sweating and puzzlement, the bias is tuned in, and the EL34's are idling at 70%



The sound was a bit edgy, and I started looking for ways to apply negative feedback. My options on the OPT was limited to unbalanced global NFB.

Next I wired up another OPT (Japanese made CALRAD 6K6:4/8/16) which gave me options to use local cathode feedback on the EL34's. That really improved the sound.




OEM manufacture of Tube Audio
Power and Integrated Amplifiers
Phono Pre-amplifiers
DIY Kits
DIY Workshops

Offline Mars

  • Commercial Member
  • Trade Count: (+1)
  • *****
  • Posts: 1,250
  • Total likes: 77
  • “The path is the goal.” Mahatma Ghandi
    • Mars Tube Audio
Re: Mono block project
« Reply #5 on: November 26, 2019, 02:05:26 PM »
Before I take the amp apart again, I took some photos of the exterior to give an idea how these will look when done:









and the star of the show: the 12BH7 dual triode


OEM manufacture of Tube Audio
Power and Integrated Amplifiers
Phono Pre-amplifiers
DIY Kits
DIY Workshops

Offline handsome

Re: Mono block project
« Reply #6 on: November 26, 2019, 06:21:20 PM »
Nice. How are you implementing your balanced inputs?

Offline Mars

  • Commercial Member
  • Trade Count: (+1)
  • *****
  • Posts: 1,250
  • Total likes: 77
  • “The path is the goal.” Mahatma Ghandi
    • Mars Tube Audio
Re: Mono block project
« Reply #7 on: November 27, 2019, 10:48:50 AM »
Thanks, handsome

Here is the schematic of the balanced configuration. I am having difficulty configuring switch between unbalanced and balanced.



This amplifier responded well to cathode feedback, so that would be implemented when the OPT's are rolled.
OEM manufacture of Tube Audio
Power and Integrated Amplifiers
Phono Pre-amplifiers
DIY Kits
DIY Workshops

Offline handsome

Re: Mono block project
« Reply #8 on: November 27, 2019, 12:27:54 PM »
just add a switch to ground the grid of the unused phase - e.g. short one of the grid resistors but use grid stops or it might oscillate. if you are only using cathode feedback thats all you need, if you are trying to implement global feedback as well the switching gets complicated best to use an input transformer and shunt feedback was my untested solution...

Offline Mars

  • Commercial Member
  • Trade Count: (+1)
  • *****
  • Posts: 1,250
  • Total likes: 77
  • “The path is the goal.” Mahatma Ghandi
    • Mars Tube Audio
Re: Mono block project
« Reply #9 on: February 18, 2020, 11:30:53 AM »
Time for an update on the mono block project.

For the moment I am running with the balanced topology. I tried driving the block with a single ended source (one input grid earthed). It worked, but the sound was uninspiring. At least the monoblock plays music.

Rather than rebuilding the frontend for single ended sources, I left it as in the circuit above, and focused on building a single channel balancer. One 6SN7 and some resistors later I have a balanced signal to drive the mono block. I also got some signal gain in the process.



Meanwhile I rolled a Quad style OPT on the biggest stack I could manage with 38mm tongue laminations. This was inspired by the thread about Quad OPT's elsewhere on the forum. Turner promotes the idea to design for 20Hz reproduction at full power, and I went with that suggestion. In the end I had a massive 50watt OPT with 10% cathode feedback and 25% screen grid taps.

The OPT is designed for 4 ohm connectivity



I connected the OPT and realised that the core is a bit unwieldy for this chassis. I might roll the next one on a 60mm stack (in stead of the 75mm stack on the photo) so that the tubes have more space around them.





The sound is very good. Effortless bass; just what I wanted. The sensitivity of the system is a bit high when using the computer sound card. When using the cell phone as a source, the gain was spot on. I changed the V1 from a 12AX7 to a 12AU7 with acceptable reduction in sensitivity.

So far so good. If I can find a 50 watt 4 ohm load, I can measure the max output of the block. Every drum beat and every bass note is clear and solid. I wonder what the amp will sound like if I add cathode feedback too?

OEM manufacture of Tube Audio
Power and Integrated Amplifiers
Phono Pre-amplifiers
DIY Kits
DIY Workshops

Offline Den123

Re: Mono block project
« Reply #10 on: February 18, 2020, 04:24:11 PM »
Interesting thread. Nice work.

Offline fredeb

Re: Mono block project
« Reply #11 on: February 18, 2020, 09:44:40 PM »
Yes , very interesting . I'm going to have to come have a listen to this amp you're developing Karel .

I love the way you're experimenting with this .
...evolution is the gradual development and stratification of progressive series of wholes, stretching from the inorganic beginnings to the highest level of spiritual creation.
Jan Smuts

Offline Mars

  • Commercial Member
  • Trade Count: (+1)
  • *****
  • Posts: 1,250
  • Total likes: 77
  • “The path is the goal.” Mahatma Ghandi
    • Mars Tube Audio
Re: Mono block project
« Reply #12 on: February 19, 2020, 08:58:49 AM »
Yesterday I connected the cathode feedback. Thus the amplifier now had ultra linear coupling as well as cathode feedback.

The circuit did not like it. The sound was good, but the driver struggled to drive the output tubes sufficiently. So I am back to no cathode coupling and only UL.
OEM manufacture of Tube Audio
Power and Integrated Amplifiers
Phono Pre-amplifiers
DIY Kits
DIY Workshops

Offline handsome

Re: Mono block project
« Reply #13 on: February 19, 2020, 09:40:14 AM »
How did the driver struggle? Cathode feedback lowers gain - are you sure your just did not have less gain?