Nisbeth, if you decide to create (lay out) a new improved printed circuit board for the ACP+, including those two trimmer potentiometers you mentioned, please permit me to offer a couple of suggestions for additional modifications that you might want to think about incorporating:
- Include a PCB footprint for an optional power on-off switch. People who don't want it can of course solder in a jumper wire.
- Include a PCB footprint for a 3.5mm TRS jack, for people who want to plug in headphones or earbuds that don't have a 1/4" plug. Do it in whatever way is easiest for you; if it means the 3.5mm jack is unswitched, no big deal.
- Include a couple of relays, one per channel, that implement switchable gain. And of course a footprint for the switch that activates/deactivates the relays. Good choices for gain might be +7dB (low gain setting), +17dB (high gain setting). But since you're the one designing the board, you get to make the final decisions about gain numbers.
Not generally. Being cap coupled, you don't really need to trim it unless
it's seriously out of spec. The amplifier is very tolerant of variations,
intentionally so.
I like big caps, and I cannot lie.
Thanks for the help, PRR!
I got to that end of the pig already: Output cap is now a 3900uF Panasonic FC. Nelson Pass is my unrivaled transistor guru, but I also know him as a multilevel entertainer and provocateur, so when he leaves empty space within the silk screening, I know he's looking for a reaction.
Sure, film caps can be pricey, but I'd been dying to try these unpretentious ones for $3, so in they went: 3uF Vishay 1848 MKP from Mouser, my favorite boutique. Pearls before swine? It's never sounded better, and the smoke is still kept safely inside.
Now, did someone mention more gain? I could use a couple of dB...
Thanks for the help, PRR!
I got to that end of the pig already: Output cap is now a 3900uF Panasonic FC. Nelson Pass is my unrivaled transistor guru, but I also know him as a multilevel entertainer and provocateur, so when he leaves empty space within the silk screening, I know he's looking for a reaction.
Sure, film caps can be pricey, but I'd been dying to try these unpretentious ones for $3, so in they went: 3uF Vishay 1848 MKP from Mouser, my favorite boutique. Pearls before swine? It's never sounded better, and the smoke is still kept safely inside.
Now, did someone mention more gain? I could use a couple of dB...
Attachments
Here is my finished ACP+. I had assembled the board at BAF2019. Thanks go to Nelson and everyone involved who contributed time and parts to make this a success.
Originally I was going to sandwich the board with plexiglas, but I decided in the end to go super budget and used what I had on hand, which was half inch plywood. I did purchase some M4 standoffs and screws, and a 2.1mm DC pcb power jack and plug to finish it off. I have also purchased an inline power switch that has male and female 2.1mm plugs that will insert into the DC power line.
I just purhased a pair of AKG K240 Studio headphones to go with the ACP+. It was only $70 CAN (available in the USA for $44 USD), so it is not the best headphones available, but I have high hopes for it.
Now these headphones are 55 ohm and the ACP+ is optimized for 32 ohm. I decided to modify the ACP+ so that it sees 32 ohm with the headphones in the circuit. I inserted 82 ohm resistors between signal out and Ground, and also replaced the 32 ohm resistors at the switched preamp outs to Ground (R16 and R17) with 56 ohm resistors. The net result is in preamp mode, the 82R and 56R are in parallel, giving 33R. With the headphones plugged in, the 56R is switched out, the 82R is paralleled with the headphone's 55R, giving 33R.
One other thing I did was I paralleled the 10uF input capacitors with 35 cent Wima MKS 1uF film capacitors. They were in my spare parts box so I threw them in.
After burning in the AKG phones with the ACP+ and FM tuner for about 80 hours, I started to listen to them at 11 o'clock last night. Fed by a TDA1541A NOS dac, I enjoyed the sound of this combination so much that I did not stop until 1:30am. The AKGs lacked extreme low bass, but overall I was very happy with the sound. I don't have any other headphones/amp to compare with, but good sound is good sound.
Today I am listening to the ACP+ as a preamp in my system and again, the sound is very good. I'm not missing my Luminaria preamp (but it will go back in).
Originally I was going to sandwich the board with plexiglas, but I decided in the end to go super budget and used what I had on hand, which was half inch plywood. I did purchase some M4 standoffs and screws, and a 2.1mm DC pcb power jack and plug to finish it off. I have also purchased an inline power switch that has male and female 2.1mm plugs that will insert into the DC power line.
I just purhased a pair of AKG K240 Studio headphones to go with the ACP+. It was only $70 CAN (available in the USA for $44 USD), so it is not the best headphones available, but I have high hopes for it.
Now these headphones are 55 ohm and the ACP+ is optimized for 32 ohm. I decided to modify the ACP+ so that it sees 32 ohm with the headphones in the circuit. I inserted 82 ohm resistors between signal out and Ground, and also replaced the 32 ohm resistors at the switched preamp outs to Ground (R16 and R17) with 56 ohm resistors. The net result is in preamp mode, the 82R and 56R are in parallel, giving 33R. With the headphones plugged in, the 56R is switched out, the 82R is paralleled with the headphone's 55R, giving 33R.
One other thing I did was I paralleled the 10uF input capacitors with 35 cent Wima MKS 1uF film capacitors. They were in my spare parts box so I threw them in.
After burning in the AKG phones with the ACP+ and FM tuner for about 80 hours, I started to listen to them at 11 o'clock last night. Fed by a TDA1541A NOS dac, I enjoyed the sound of this combination so much that I did not stop until 1:30am. The AKGs lacked extreme low bass, but overall I was very happy with the sound. I don't have any other headphones/amp to compare with, but good sound is good sound.
Today I am listening to the ACP+ as a preamp in my system and again, the sound is very good. I'm not missing my Luminaria preamp (but it will go back in).
I decided on this simple case because firstly, it is simple, but mainly because it allowed me to make use of the pcb mounted jacks, switch, and pot without having to carefully align holes to a case front and back, or needing to provide additional switch and jacks on an enclosed case.
However, the idea is not original. Most notably, the late Don Garber (Fi Audio) cased his tube preamps and SET amplifiers in similar fashion, except he used aluminum plate.
As mentioned previously, I am also adding an external inline switch so I can turn the ACP+ on and off without unplugging the power. It plugs into the 2.1mm power plug and jack that I installed.
However, the idea is not original. Most notably, the late Don Garber (Fi Audio) cased his tube preamps and SET amplifiers in similar fashion, except he used aluminum plate.
As mentioned previously, I am also adding an external inline switch so I can turn the ACP+ on and off without unplugging the power. It plugs into the 2.1mm power plug and jack that I installed.
Attachments
