Amp Camp Pre+Headphone Amp - ACP+

[WalterW]

@WalterW Very nice, and thanks for confirming that a Toroidy transformer with their mounting plate will fit in a 2U enclosure. I am using one about the same size (100VA) for a UGS Muse preamp and was worried it might be too tall. Did you use 10mm standoffs, or something shorter? Also, did you make your own pushbuttons or did you find them somewhere?

Hi Pars, I used the standoffs which were delivered by Toroidy. I think it was 10mm M4.

For the pushbuttons I used this type:

If you search 'push button knob 10mm silver' on Ali you will find them.
Together with this 'Tactile micro switch':

Cheers, Walter

 

[WalterW]

Walter, this is very nice as always.
Could you describe the difference in sound character between ACP+ and B1 r2 in a few words?

Hmmm, at first glance I hear no difference. But I haven't done any serious listening tests yet. Will do coming holidays with Christmas and so on. 🙂

 

[Dan K]

I've an ACP+ Pre and I'm wondering if I should change the values of R16 & R17 to perfectly drive a pair of AKG K340?

These are the 33 Ohm headphone output loading resistors. Any suggestions on an ideal value would be very helpful

Thanks

 

[cubicincher]

Hello Dan K,
check post #325 of Ben Mah - especially his pic with the different values for the Rs.
Greets
Dirk

 

[Dan K]

Forgive my ignorance, but is the tuning of R16 &17, and the suggested bypass resistor in post 325 simply to ensure that the output to the power amp is at the same level as the phones?

Does this mod change the ACP's ability to drive the headphone in any way?

I've got AKG K340 and K240 and just want to be sure I'm getting the most out of them that I can in this setup

Many thanks

 

[cubicincher]

The AKG 340 has a nominal impedance of 400 Ohm.
The AKG 240 has a nominal impedance of 50 Ohm.
This is what the ACP+ 'sees' at its output. This is a preamp - but it can drive headphones. Originally it was designed for circa 32 Ohm as an ideal load - impedance. This should be 'much' lower than the input impedance of a poweramp. Many poweramps have an input-impedance anywhere between 10kOhm and 100kOhm (sure there are exceptions). No problems here.
With your headphones the situation changes for the ACP+. You can get problems to drive it properly. Bass-rolloff could appear and other symptoms caused by the impedance - mismatch.
So I would change the Rs according to Ben Mahs recommendations. Perhaps making it switchable for the two headphones.
But first I would build the ACP+ and I would listen with those AKG headphones. Test, test and let your ears decide.
Cheers
Dirk
p.s.. my ACP+ drives my headphones with ease (for example a Beyerdynamic DT880 Pro with 250 Ohm)
Greets
Dirk

 

[CWelsh52]

@cubicincher


p.s.. my ACP+ drives my headphones with ease (for example a Beyerdynamic DT880 Pro with 250 Ohm)

Are you using the originally specified resistors at R16 & 17, or did you change them to match your 250 Ohm phones?

 

[cubicincher]

Hello CWelsh52,
I have changed the resistor values to match the 250 Ohms of the Beyerdynamic cans. But it also worked really nice with the original R - values.
Cheers
Dirk

 

[CWelsh52]

^^ Thank you. I don't listen with headphones often, but mine are 64 Ohms so they ought to be fine with the original resistors.

 

[CWelsh52]

I received my ACP+ kit today and discovered several of the resistors do not match the values specified in the list of supplied parts. I don't know if the values have been changed somewhere along the line and the list has not been updated, or if I was actually shipped the wrong parts. I hope someone can guide me to the answers.

Resistor Position -- Specified Value -- Measured Value
R 16, 17 -- 33.2 Ohm -- 36.5 Ohm
R 12 -- 2.2 Ohm -- 10.2 Ohm
R 13 -- 6.8 Ohm -- 5.6 Ohm
R 18, 19, 20, 21 -- 1 Ohm -- 4.3 Ohm

Thanks in advance!

 

[6L6]

Did you measure them? Or are you reading the codes? Post a photo or two please.

Every value you mention is low ohmic, I suspect your multimeter has trouble reading values in that range - which is very common...

 

[CWelsh52]

@thomasnadeau Please see post 2,670 above

I am measuring them. And, yes, mine seems to struggle with the lower values. What would you like in the way of pictures? Do you want the resistors up close, or pictures of the reading on my multimeter?

 

[Halauhula]

I believe photos of the resistors with the color codes clearly visible is what is needed.

 

[CWelsh52]

These aren't great. I hope they help. The picture of R 12 is particularly bad, but even under my magnifying glass it is hard to read.

 

[CWelsh52]

And, really, everything might be just fine with what I've received. The values just don't match the specs in the original design or the list provided in the package, so I just want to make sure I'm okay before I start soldering. 🙂

 

[CWelsh52]

Reading the codes, it appears that everything matches the specified values. Maybe it is just a problem with my multimeter. This is where my ignorance holds me back. Do I trust the codes and ignore measurements for the very low values? Or, is there something I need that can measure more precisely?

By the way, the codes indicate that I have R 12 and R 13 reversed in my identification.

 

[Mark Johnson]

I gave a presentation at Burning Amp 2024 a few weeks ago, in which I recommended this multimeter. It's a handheld (battery powered) meter whose display has 4.5 digits, basic accuracy is 0.1%, and it'll measure 0.1 millivolts and below. A very nice value IMHO and a good fit for the topic of my little talk: transistor matching.

Hoping this is helpful.

 

[6L6]

Trust the codes. 😊

Get a better meter if you think you’ll be doing more of this, but it’s not strictly necessary.

 

[CWelsh52]

I do expect to be doing more of this. I will be checking out Mark's recommendation.

Thank you, both, for the help.

 

[Halauhula]

Trust the codes.