The tale of two amps

[Tripmaster]

Hi

I purchased my first Dual Mono LM3886 based amp from Chipamp.com

Shortly after making the amp my father came to stay for a few days and after hearing it, he wanted to buy from me. Since then his components have been lying in a box disconnected.

I decided to build another amp, but this time I sourced the components from Farnell and also built a Pedja Rodic discrete power supply.

I've been running my amp for a while and have been quite happy, but its always sounded a little bit flat. It wasn't until I connected my dads amp, and ran them side by side did realise his sounded so much better. The higher frequencies were there in abundance and it sounded dynamic as a result. Its surprising how you get used to something after a while!

Below I have listed my fathers components in black, and mine in red. Its obviously better to change the specification to match his but I wondered if there is anything in particular I could change to prevent the higher frequencies rolling off so soon?



Amp PCB:

R1 - input resistor = 1k (Installed 996R)
R2 and Rf - input-to-ground resistor = 22.1k (Installed 21.3K)
R3 - feedback-to-ground resistor = 681 (Installed 678R)

Rm - mute resistor = 10k (Installed 9.94K)

Rz - 2.7 ohm 2w resistor for zobel network (Installed 2.9R)

C1,C2,Cm – 100uf Panasonic FC (Same)
C3,C4,Cz – 0.1uf BC Components (100n BC 0845F Components)
Ci – 47uf Panasonic FC (Same)

I used Welwyn RC55C 1/4w resistors for R1-R3 and Rf.
Rz is a 2 w Vishay Metal Film

PSU:

Standard Unregulated Snubberized 36v (My amp has the Pedja Rodic Discrete regulated running at 27v)

Transformers 2 x 300VA 2x25V Secondaries (Same)



N.B the attached pictures are of my amp. I purchased the PCBs separately

Any help would be appreciated!

Regards

[Piggy]

sounded a little bit flat.... swap the power supply and see if it sound any better

[wahab]

the regulation seems much weak on the photo, and there s no
big caps in the regulated side...
anyway, theses chips don need regulations, but rather big supply
line caps..yours seems ok, but the regultion create a limitation in
peak currents..

all da best,

wahab

[Conrad Hoffman]

It can be a bit risky to narrowly define a problem like high frequency rolloff, since problems in other frequency ranges can convince you there's something wrong with the highs. If I understand right, the "good" amp had unregulated supplies, whereas the less good amp used regulated supplies. I've never compared, but have read that power amps with unregulated supplies generally sound better. I can imagine a variety of reasons for this, dynamic headroom being the biggest, instantaneous current capability the second. The first thing I'd do is try your amp with the same type of supplies the "good" one used. After that, I'd look at the ground and power wiring to be sure they were done exactly the same way.

CH

[Tripmaster]

Originally Posted by wahab

the regulation seems much weak on the photo, and there s no
big caps in the regulated side...
anyway, theses chips don need regulations, but rather big supply
line caps..yours seems ok, but the regultion create a limitation in
peak currents..

all da best,

wahab

Hi Wahab

The power supply is built as per Pedja's schematic.



I failed to leave enough space at the end of the strip board for the 100uf caps, so I inserted them on the power supply output terminal blocks. At the time I didn't think it would make any difference.

[Tripmaster]

Originally Posted by Conrad Hoffman

It can be a bit risky to narrowly define a problem like high frequency rolloff, since problems in other frequency ranges can convince you there's something wrong with the highs. If I understand right, the "good" amp had unregulated supplies, whereas the less good amp used regulated supplies. I've never compared, but have read that power amps with unregulated supplies generally sound better. I can imagine a variety of reasons for this, dynamic headroom being the biggest, instantaneous current capability the second. The first thing I'd do is try your amp with the same type of supplies the "good" one used. After that, I'd look at the ground and power wiring to be sure they were done exactly the same way.

CH

Hi Conrad

I took Nick's advice regarding the power supply as it was his preferred choice.

Building a buffered Gainclone chip amp.

I would rather not remove the unregulated ps from my fathers amp, but I could easily knock one up. I just wanted to check there wasn't anything obvious with my resistor values before I put together another Farnell order.

Thanks

[wahab]

Originally Posted by Tripmaster

Hi Wahab

The power supply is built as per Pedja's schematic.



I failed to leave enough space at the end of the strip board for the 100uf caps, so I inserted them on the power supply output terminal blocks. At the time I didn't think it would make any difference.

the four caps at the input of the regulation are ok, but the two 100uF
at the output are not enough..you can replace them with 4700uF if you have them, if not, remove two from the inputs and connect them to the outputs,
and check the difference..

[Tripmaster]

Originally Posted by wahab

the four caps at the input of the regulation are ok, but the two 100uF
at the output are not enough..you can replace them with 4700uF if you have them, if not, remove two from the inputs and connect them to the outputs,
and check the difference..

Hello again

I don't think I have any, so if I were to order some capacitors would 4700uf be the optimal value? I also have 2x 100uf at the chip.

[wahab]

2 X 2200 uf per channel, at least, as it seems that you have separate regulations
for each channel...that make four caps to add..

[Tripmaster]

Originally Posted by wahab

2 X 2200 uf per channel, at least, as it seems that you have separate regulations
for each channel...that make four caps to add..

OK thanks. Can you see anything that needs changing on the amp pcb? Are the resistor values within tolerance?