[macky112]

Hi @xrk971 could you help me understand the I/V, LPF, AMP and BUF stages of an amp and how rolling different opamps at each stage may affect sound?

 

[xrk971]

I am trying to avoid opamps. They add many many transistors to the signal path. They are not Class A in general.

I/V is current to voltage converter. Buffer is a unity gain amp and generally provides lots of current but no voltage gain. So you drive it with a good preamp and as a "master" and the current buffer does what the master tells it.

The Pocket Class A is a good preamp as it can drive loads as low as 300ohms to 10v peak to peak. Add a current buffer capable of several amps and you can drive most speakers to a decent volume.

When you say rolling opamps it generally means a power amp with an opamp front end that's interchangeable. Most opamps nowadays are all very low distortion and neutral. They use lots of negative feedback and have a harmonic profile that may be low by not pleasing like SE Class A.

So I guess short answer is, use this Pocket Amp as a front end to any of your other amps and it's like "rolling a SE Class A" opamp into your system.

 

[macky112]

Another question is there any benefit between 16v power supply vs +\- 8V?

 

[xrk971]

On a single ended Class A amp the ps is single voltage. On a push pull amp the supply is dual rail. Is there advantage with dual rail ? Well it doesn't use caps in output path. Some people don't like caps but I don't have a problem with it.

 

[macky112]

On a single ended Class A amp the ps is single voltage. On a push pull amp the supply is dual rail. Is there advantage with dual rail ? Well it doesn't use caps in output path. Some people don't like caps but I don't have a problem with it.

I see. Thanks!

Then if is it a single voltage ps for push-pull amp then its not good right?

 

[xrk971]

You can't have single rail on push pull - at least not without an output cap.

 

[macky112]

You can't have single rail on push pull - at least not without an output cap.

Ah I see ok, maybe that's why ppl weren't very fond of single rail ps on those budget dap threads that I frequent because of the output caps!

 

[gwompki]

I'm going to go ahead and post a mini review thats focused primarily on the Sennheiser HD-6XX (aka HD-650). I only have about 4 hours on it at this point so these are early impressions.

I have tried this headphone from a variety of budget amps(Little Bear B4, Fiio E12A, Little Dot 1+, and an old 70s integrated amp...also straight from the output of my LG V20 which is supposedly adept at driving these phones according to Jude). I had been happy listening to vinyl with these phones through my old integrated amp as it has gobs of power. However, when using a digital source everything felt way too laid back and warm. All of the other amps either felt too warm or the low end wasn't present enough, etc. I had grown accustomed to listening straight from my V20 but I always thought I was missing out on something....low end, sound stage, etc.

Now here comes the Pocket Class A (PCA). Using the V20 as a source my HD-6XX came alive. I'm still trying to find the words to describe what I'm hearing when A/Bing between the LG's output and the PCA. It is very noticeable...all of the instruments seem to spread way out and I can clearly identify each one. The low end is much more coherent and tangible to a certain extent. Everything is much more authoritative in general. I have noticed that more power will make the HD-6XX sound less congested, but none of my other amps offer the same level of dynamics that the PCA does. Suffice it to say I am enjoying this little beast immensely!

I have only spent a small amount of time with the Fostex TH-X00 and this amp so far as they are easy to drive. However, early testing is also very positive. This amp seems to really make them shine. Can't wait to spend more time with them.

 

[stellarelephant]

Nice work on your amp, Gwompki.
I'll agree...it's so nice to to hear headphones adequately powered!
I notice an immediate difference with every can I've tried with the PCA, but I'll bet the effect is huge with the HD6XX.

I too thought that surface mount soldering was hard until I watched this tutorial.
You already made it through, but this demonstrates the technique X described:

 

[stellarelephant]

Update on my first build:
I ended up soldering in the 35V 47uF Silmics as bypasses for the 25V 470uF Nichicon KA output caps. This added some bass heft and treble clarity, and yielded a smoother and more neutral sound signature over the KA alone, which I felt was a bit midrange-centric by itself. It's a great sounding amp that I have been listening to constantly with headphones, as well as using as a preamp for my car stereo and full-range mini system.

I've recently discovered something special about this amp: it really likes to be driven hard with a hot signal. Surprisingly enough, I find it sounds even more revealing when I feed it the amped headphone signal from my DAP, instead of using the auxilary output. It also rocks with the powerful signal I get out of my Firewire audio interface, which is noticeably louder than a standard line out, and is intended for balanced studio monitors (though I'm using it single-ended). I did not expect this effect, and I am curious if anyone else agrees.

 

[stellarelephant]

And now for my second build! This time around, I pulled out all the stops.

-All resistors are 1/5 Watt Vishay MELF
-Higher bias via R4 value of 51R
-Bit higher dummy load via RL value of 1K
-Power rail caps are 25V 1500uF Panasonic FR
-Power bypasses are original value, but TDK C0G
-Input caps are my DIY stacked and leaded 2x 63V .68uF WIMA SMD-PPS
-Still auditioning output cap options (about to order more)



Powered by my OKCell USB batteries, this amp measures 7.5VDC (divided by the R7 parallel array value of 117.5R, times 1000 = a toasty 63.8 mA bias current!) I have only had time to listen through one battery cycle, but I can say for sure that dynamics and detail are just superb.

I auditioned my DIY PPS input caps vs. the stock polyesters, and quickly heard a difference (hopefully not my subjective bias

). The polyesters have a creamy sound with a lot of bloom and soft treble. The PPS opens up the sound significantly with better precision, speed, and especially more air--exactly what I was hoping for! I soldered them in without hesitation. They require some sanding of the inner side of the input jack to slide in, at least with the caps shrink-wrapped like I did.

I need to place an order for more output caps to experiment with, but right now I am loving the sound of this amp with the trusty 25V 470uF Nichicon KA standing alone on output duty. I want to use this amp on the go in a tin, so I am exploring other options that will fit. So far my Mouser cart has three other Nichicon audio grades-- FW, KW, and FG (Fine Gold...high hopes for this one!). I am also going to order the big 16V 470uF Silmics that Funch used, and also X's current favorite cap combo.

I also borrowed a pair of ATH M-50s from a friend, so I can really judge bass quality better going forward with my listening tests.

 

[xrk971]

I'm going to go ahead and post a mini review thats focused primarily on the Sennheiser HD-6XX (aka HD-650). I only have about 4 hours on it at this point so these are early impressions.

I have tried this headphone from a variety of budget amps(Little Bear B4, Fiio E12A, Little Dot 1+, and an old 70s integrated amp...also straight from the output of my LG V20 which is supposedly adept at driving these phones according to Jude). I had been happy listening to vinyl with these phones through my old integrated amp as it has gobs of power. However, when using a digital source everything felt way too laid back and warm. All of the other amps either felt too warm or the low end wasn't present enough, etc. I had grown accustomed to listening straight from my V20 but I always thought I was missing out on something....low end, sound stage, etc.

Now here comes the Pocket Class A (PCA). Using the V20 as a source my HD-6XX came alive. I'm still trying to find the words to describe what I'm hearing when A/Bing between the LG's output and the PCA. It is very noticeable...all of the instruments seem to spread way out and I can clearly identify each one. The low end is much more coherent and tangible to a certain extent. Everything is much more authoritative in general. I have noticed that more power will make the HD-6XX sound less congested, but none of my other amps offer the same level of dynamics that the PCA does. Suffice it to say I am enjoying this little beast immensely!

I have only spent a small amount of time with the Fostex TH-X00 and this amp so far as they are easy to drive. However, early testing is also very positive. This amp seems to really make them shine. Can't wait to spend more time with them.

Gwompki,
Thanks for youy listening impressions. Nice to hear that it gave you a sound that was lacking in all your other amps. The HD-6XX sound like they were made for this amp. The lowish bias current will favor high impedance cans that's for sure. But to hear that instruments opening up and more coherent and authoritative bass makes my day. There really is something special when an amp uses very few components and is SE Class A.

 

[xrk971]

StellarE:

I've recently discovered something special about this amp: it really likes to be driven hard with a hot signal. Surprisingly enough, I find it sounds even more revealing when I feed it the amped headphone signal from my DAP, instead of using the auxilary output. It also rocks with the powerful signal I get out of my Firewire audio interface, which is noticeably louder than a standard line out, and is intended for balanced studio monitors (though I'm using it single-ended). I did not expect this effect, and I am curious if anyone else agrees.

I think I notice the same thing, I usually set my sources quite loud, maybe 70% of max.

Great work on your second amp. Looks very cool and powerful.

 

[stellarelephant]

I've been playing around with the limited capacitor stock I have at the moment.
Early impressions rolling output caps with my new amp:

25V 1000uF Nichicon KA - Good bass, forward and slightly hot mids, soft upper treble lacks some air. Nice balance between detail and euphonic warmth and presence.
25V 1000uF Panasonic FR - Neutral/analytical, with powerful deep bass and great airy treble extension. Cooler mids. Brighter presentation, lively dynamics.
16V 1000uF Panasonic OSCON - Perfectly neutral response, with tight bass control. Pleasantly smooth, integrated, somewhat distant sound. Soft treble details and restrained dynamics.
25V 47uF Nichicon KZ - Not nearly enough capacitance for bass with my low-R cans, but wow. Wonderfully clear and engaging mids and treble. If I only had the 300 ohm HD600...

 

[gwompki]

I've found a weird issue on my build...I bought some EBL rechargeable 9V batteries from Amazon that arrived yesterday. I charged them overnight then unplugged and set aside this morning. I had been using Duracell 9V with no problem for the past week. Tonight I suspected the Duracell were getting weak so I swapped in two of the EBL's and....no sound. Thought maybe I had pulled out one of the battery snap leads so I swapped back in the Duracell to test and the sound was back. Thought maybe the EBL didn't charge properly so I measured all 4 at 8.4V. Couple of other symptoms...LED lights up but seems to be a bit less bright, Also when I turn it on I hear a tiny click out of the right driver as opposed to the louder whoomp I hear from both channels when I power it on with the Duracells.

Is it possible that all 4 EBL batteries are bad or is there something else at play here? Haven't had a chance to take voltage readings on the circuit yet. Might need some guidance on that if it comes down to it.