Sgheadphones DIY A47 pocket amp kit

[tropicalrips 0]

i'll bring both

[N@Z 0]

Finally! Looks good. Looking forward to the imitial impressions.

[tropicalrips 0]

Jason hav u decided on a name for this amp/board yet?

any suggestions?

[bpribadi 0]

I like those translucent casing... Where did you bought it ? Looks cool

 

Very neat layout Tropical

[tropicalrips 0]

I like those translucent casing... Where did you bought it ? Looks cool

 

Very neat layout Tropical

thanks

the case is from Farnell, its a hammond, there is also a fully transparent type of the same size

[jasonhanjk 0]

On the board. Shhh, secret ok...

[heady 0]

Will someone please post opinions of the audition? TropicalRips?

[tropicalrips 0]

it sounds very close to my clone, without using the more expensive AD8620 op-amp.

low frenquency is good, has a bass kick on SR80 but not boomy (similar to clone)

i think it may not be as detailed as the clone, but its close, very hard to tell, sounds a little more "laid back" or slower

need another pair of ears to confirm

[nakedtoes 0]

[bpribadi 0]

Thanks to Jason, he gave me one of his PCB for me to test it.

 

So I just finished it yesterday, and I would say, as a budget amp, it sounds good!

 

I tried the one that Tropical made, but it was a short time listening, so not very objective listening. It has a nice midrange and bass (Tropical put very good coupling caps on the input), but sounds laid back, and a bit less in detail compare to a normal CMoy. Need to listen more when time permit. This time compare with mine

 

So after analyzing the schematic, I think from my perspective, the ground channel can be improved and simplified. So instead of using the original ground channel schematic from Jason, I did a minimum and simple modification, so I can use the high output current LM6171 (rated 135 mA output current). The LM6171 is a wonderful Op-Amp and priced about the same as the OPA2134, so there will be no significant change in the cost for the whole amplifier.

 

Here is the amp:

 

 

 

I use Panasonic FC capacitors, OPA2134 for the L&R channel, LM6171 for ground channel, and no input capacitor. So I made it DC coupled amplifier for maximum BASS

 

So here are the modification I made:

1. Bypass the input caps (C1 & C2) with jumpers.

2. 1k8 ohm for the R13 & R14 (voltage divider).

3. 1k ohm for the R4 & R9

4. 7k5 ohm for the R3 & R8 (feedback resistors) to set the gain at 8.5.

5. 12 ohm for the R1, R2, R6, & R7 (I think 10 ohm is enough, but I don't have it. 47ohm is too high).

6. R12 change to jumper, 560 ohm for R11 (to improve stability of the ultra high speed LM6171).

7. LM6171 for the ground channel.

 

To use LM6171 for the ground channel, you can see from the foto, I remove one of the PCB track between pin 6 & 7 of the ground channel Op-Amp, and jumper pin 7 & 8. That's it, a very minimum modification, but significant. Guaranteed no more 'far' or laid back sound caused by the amp circuit (only the slight signature laid back sound of the Burr Brown Op-Amp).

 

 

The reason for those modifications are:

1. Improve bass by removing input caps.

2. I don't have resistor values suggested in Jason schematic.

3. Improve ground channel.

 

I tested it with Grado RS-1 (32 ohm), Beyer DT-880 (250 ohm), and Sony MDR-E888 (16 ohm). With gain 8.5, it can drive DT-880 loud enough (enough for me), even though you can turn the pot to the max without being too loud. For RS-1 gain 6.6 is already very loud, so 8.5 is more than enough.

 

Powered with 2x9V batteries, the A47 configuration made this amp powerfull enough to drive most of headphones. The effect of little different of phase of the Op-Amp 1 output and the Op-Amp 2 (buffer Op-Amp) is very minimum, and I would say not really audible.

 

Rolled the OPA2134 with OPA2107 gave a marginal improvement. OPA2107 refine the sound of the whole amplifier. The midrange sounds more neutral, and a better overall sound. But not recommended for budget amp, OPA2107 cost almost 5 times the OPA2134.

 

 

So my recommendation is, the cost for the input caps is allocated to change the OPA2134 to OPA2228 (provided your source has a very low DC output). I measured DC offset less than 15 mV which is very safe for any headphone. You won't notice this offset with high impedance headphone such as DT-880, and it is just a very slow 'pluck' sound with the low impedance headphones. Personally I don't fan OPA2134, therefore I recommend to change it with OPA2228 (personnal taste).

 

To lower down the DC offset, the Input ground can be put on the ground channel output, directly connected to the Ouput ground. In Jason schematic, the Input ground is connected to the ground channel input.

 

 

Recommended budget amp! Congrats Jason!

Edited November 8, 2004 by bpribadi

[heady 0]

tropicalrips, I have been looking for a case like the cool blue transparent Hammond in the pic you posted on 17 Oct. I can't find it in Farnell. Please tell me you got it in Singapore...

[tropicalrips 0]

bram, nice it''ll be interesting to compare our 2 amps, maybe a mini meet? i think jason has also completed his amp, his should be the original unmodded version

 

heady, i got the Hammond case from Farnell, i don't hav the part no. now will need to look, but i hav no. for the same but smaller case 381-8743 hope this helps.

 

i remember using the Hammond part no. to search Farnell website to find the case cause the site's search function is

 

currently doing a mod on a pocket radio hav changed out 3 capacitors, not sure if it actually improves the sound, but it's driving my HD595 now, not bad for a $18 radio

[N@Z 0]

Cool, how about a new thread with some pics on the pocket radio mod?

[bpribadi 0]

it''ll be interesting to compare our 2 amps, maybe a mini meet?

 

A mini meet would be cool

 

Let me know the convenience time, we can have dinner in SLS while evaluating Jason's amps

[heady 0]

I second bpribadi, only please make it next weekend, I have conference away until Friday.