I'm excited to share my first ever full amplifier build: A headphone amp made from the RH84 circuit! I've built kits and restored vintage amps, but never built one from scratch like this. I chose this build because it was simple enough for a first-time build, and it presented a lot of opportunity for learning. I learned so much from this project, and it definitely set a good knowledge baseline for more involved projects down the road.
RH84 circuit: RH Amplifiers: RH84 amplifier - revision 2
I have a tendency to crave doing things in a way that makes them unique... so I made the following changes to the design:
1. Modified the power supply to use two 6AX4 damper diodes instead of a more "standard" rectifier.
2. Added volume control.
3. Added a switch to switch between pentode and triode-strapped mode for the EL84s.
4. This is a headphone-only amplifier. I listen almost exclusively to headphones, so I didn't care so much about speaker outs.
Some specs:
- OPTs: custom by electraprint with 32 and 120 ohm secondaries.
- PT: Lundahl LL2741
- Separate headphone jacks for 32 and 120 ohms.
- PSU is CLCRC.
- Miflex coupling caps.
- 12AT7 driver can be swapped out for a 12AU7 for less gain and improved noise floor at the cost of some power. Power loss not really an issue for headphones.
- Chassis is from Landfall Systems
Overall I am so pleased with how this turned out! It sounds amazing and is extremely quiet. Quiet enough for headphones. The inside is a little messy and some of the wire colors don't make sense because I ran out (like the rainbow of colors going to star-ground), but overall I'd say it's not bad.
Best combinations of outputs and headphones I've discovered so far:
- Pentode mode and 32 ohm jack with hard to drive planars (Hifiman Arya). My Arya sounds incredible with the high output power.
- Pentode or triode mode with 32 ohm jack for more sensitive high impedance dynamic headphones (ZMF).
- Triode mode with 120 ohm jack for high impedance dynamic headphones (ZMF)
Enjoy!
RH84 circuit: RH Amplifiers: RH84 amplifier - revision 2
I have a tendency to crave doing things in a way that makes them unique... so I made the following changes to the design:
1. Modified the power supply to use two 6AX4 damper diodes instead of a more "standard" rectifier.
2. Added volume control.
3. Added a switch to switch between pentode and triode-strapped mode for the EL84s.
4. This is a headphone-only amplifier. I listen almost exclusively to headphones, so I didn't care so much about speaker outs.
Some specs:
- OPTs: custom by electraprint with 32 and 120 ohm secondaries.
- PT: Lundahl LL2741
- Separate headphone jacks for 32 and 120 ohms.
- PSU is CLCRC.
- Miflex coupling caps.
- 12AT7 driver can be swapped out for a 12AU7 for less gain and improved noise floor at the cost of some power. Power loss not really an issue for headphones.
- Chassis is from Landfall Systems
Overall I am so pleased with how this turned out! It sounds amazing and is extremely quiet. Quiet enough for headphones. The inside is a little messy and some of the wire colors don't make sense because I ran out (like the rainbow of colors going to star-ground), but overall I'd say it's not bad.
Best combinations of outputs and headphones I've discovered so far:
- Pentode mode and 32 ohm jack with hard to drive planars (Hifiman Arya). My Arya sounds incredible with the high output power.
- Pentode or triode mode with 32 ohm jack for more sensitive high impedance dynamic headphones (ZMF).
- Triode mode with 120 ohm jack for high impedance dynamic headphones (ZMF)
Enjoy!
Attachments
Nice amp! I like the add-on options, rectifier, mode switch, and the dual jacks are nice to not have extra switching contacts for hi-lo z. Have you also tried 12AY7 (u 44) or 12BH7?
12BH7 (u 17)
12AU7 (u 20)
12AT7 (u 60)
12BH7 (u 17)
12AU7 (u 20)
12AT7 (u 60)
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Thanks! I haven't tried those tubes, but they look like good candidates for headphone use with their lower u values.
12AY7 was meant to be a low noise and specifically "designed-for-audio" tube, mic preamps, etc. it would be interesting to compare.
