[Navyblue]

Is it advisable to build a PIMETA with the voltage gain stage bypassed, essentially turning it into a unity gain buffer?

With the above configuration, if the volume pot is removed, do I need to put resistors in its place?

My intention is to build a portable amp for driving efficient IEMs with the least amount of component in the signal path to minimise signal degradation. Would it be a good idea in comparison of conventional design? Would dynamic range suffer?

Btw, is PIMETA board discontinued? I don't find it on Tangent's shop. Or is v2 conming?

Thanks.

 

[MASantos]

V2 is going through prototipe fase.

If you just need a buffer, you could go with JISBOS, a discrete diamond buffer derived from an AMB design.(if I remember correctly)

Even IEM's need voltage gain, so what are the reasons behind wanting to not use VAS in your amp?

 

[Navyblue]

It'll be for portable use so I'd like the form factor to be as small as possible. I could be wrong but JISBOS with power supply looks like it's much larger than the PIMETA board?

Does IEM really need voltage gain? Since most if not all DAPs is already capable of deafening volume on their own?

 

[MASantos]

High Volume doesn't mean high quality amplification.

I think that if you build a pimeta with low gain you'll have a great amp! If you want it even more portable, build a mini 3!

 

[amb]

A pimeta without opamps means that you'd be running the output buffers in open-loop. The negative feedback loop(s) will both be disabled. This causes high output DC offset, high output impedance, increased distortion, reduced stereo separation, and other issues.

 

[NelsonVandal]

IMO open loop unity gain buffers have too high output impedance to do the job properly driving 16R phones like IEM's. You'll get lots of odd order harmonics.

If you're planning on trying it anyway, be sure to use the ground channel as it is, opamp and buffer. High output impedance in the ground channel will give you an unnaturally wide soundstage with a hollow mid.

IEM's doesn't need voltage amplification but adequate current and low output impedance.

A modified JISBOS suitable for lower power supply is a good idea. It could be housed in a mint tin with AAA's. I've been thinking of doing something like this if more people are interested. It could have optional voltage gain to better suit full size phones.

 

[Navyblue]

Thanks AMB and Nelson.

I am not insistent on the PIMETA. I was initially planning on building a buffer circuit based on the example in the BUF634 datasheet and thought that it'd look like a PIMETA without the opamp.

Can the PIMETA be modified to solve the issues mentioned above? Or using BUF634 is just a bad idea?

I guess I'd look into JISBOS.

 

[NelsonVandal]

Unfortunately you need some kind of gain and feedback to get a low enough output impedande to drive low impedance phones. I say unfortunately since transistors seem to sound almost transparent when used in common collector mode = emitter follower = no voltage gain, compared to common-emitter or common-base. Either you have to use global feedback as in PIMETA, JISBOS and most amps, or local feedback as in CFP (complementary feedback pair) used eg in CKK3.

Open loop diamond buffers (BUF634 is an open loop diamond buffer) sound very good driving medium or high impedance headphones at moderate listening levels. I have to admit I haven't done any extensive listening tests with IEM's. Why don't you just try it. But... like Amb said, beware of DC-offset and always measure DC from output to ground before you connect your phones. There's no easy way to adjust DC, but you could use a DC-servo to get rid of the output DC using a JFET opamp, but then the circuit gets complex and you need an input cap.

 

[tangent]

If it's true that you don't need gain, you can just leave the R3s out and jumper R4 to put it into unity gain mode. The op-amps will still have a job to do, correcting the distortions that would be caused by the high output impedance of the buffers.

Or, instead of trying to remove the op-amps, you could remove the buffers instead, turning the PIMETA into a glorified 3-channel CMoy amp. Off the top of my head, you'd jumper from pin 3 to pin 6 on each jumper, jumper R5, leave out R6, and configure R3 and R4 normally for the gain you want. (Again, you can still get unity gain by shorting R4 and leaving R3 out.) You might want to seek out uncommonly strong op-amps for this, since they'll be driving the headphones directly.

These instructions are based on PIMETA v1. PIMETA v2 can do the same things, but the instructions will change slightly. For instance, there's a feature that lets us short the buffer a little more easily.