Ber A500 Tweaks..

[Cyber_Murphy]

Just thought I'd post my new adventure!!

I'm already quite satisfied with my A500's, but in the interests of DIY & electronics, I thought I'd try a couple of mods and see how much of an impact they will have..

There a lots of DIY tweaks on offer for the A500's, I have chose 2 that are low cost and fairly easy to perform..

1) Bypass volume pot and hard wire the amp "fully open" (max vol). The A500 uses low-grade volume pots. The signal cable runs from the front panel right next to the mains power supply and the power amp module to the rear input section.

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2) Replace Opamps. The A500 uses low-grade JRC4580's. I have chosen use LM4562-MA's, just because they're cheap, readily available and perform quite well..

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I plan to do one mod at a time and I'll report my findings!!

Cheers,

Murphy

[Keith_W]

Good luck with your mods. How are you going to replace those op-amps, they look tiny! I am so ashamed, my soldering skills are so poor that I can not even join two lengths of wire.

[Cyber_Murphy]

Good luck with your mods. How are you going to replace those op-amps, they look tiny! I am so ashamed, my soldering skills are so poor that I can not even join two lengths of wire.

Hi Keith,

They're about the smallest I can do myself.. I use a fine tipped iron on low temp. I've done that size a few times and I usually find it ok.. It gets tricky if there are other components in the surrounding area and you have to start removing things to have proper access! But in this case it should be ok.. Any smaller (like my DAC chips on my DCX, which I haven't done) and I wouldn't attempt to do it myself :-)

Cheeers..

[Mika75]

You will lose the bridging functionality when removing the trim pots, and after all this ...will there be any measurable differences?

Also from what I have read it's the power supply that is the limiting factor with the A500.

[Keith_W]

You will lose the bridging functionality when removing the trim pots, and after all this ...will there be any measurable differences?

If there are no measurable differences, does it mean that no difference exists?

[Cyber_Murphy]

You will lose the bridging functionality when removing the trim pots, and after all this ...will there be any measurable differences?

Also from what I have read it's the power supply that is the limiting factor with the A500.

Hi Mika75,

I suppose I should have been more precise in my opening post.. I currently use the A500's in mono bridge mode.. So, the left channel I will hard wire fully open (vol max), and the right channel I will hard wire fully closed (min vol)..

I think not having a low grade vol pot with such a long cable run will reduce noise pickup, and reduce signal degradation. Replacement op amps should definately help, the stock chips are quite ordinarily spec'd.. From the account I hace read there is a noticable improvement in performance by swapping out the stock chips..

As for the power supply,, there are plenty of mods that tweak the power supply too!! But most require retro-fitting the case in some way for more filter caps to be fitted.. And the power supply mods help at full bandwidth, especially bass, under high output or high transient conditions.. Mine are only used from 90Hz -> 1.20KHz in an active speaker setup.. But yes, I did consider the power supply tweaks, and I probably would have gone with them if I was using the amp at full bandwidth..

Cheers..

[Mika75]

Oh ok, well I have run in both stereo and bridged mono, I wouldn't want to lose that capability.

What I believe the Behringer does lack is a simpler faceplate, incorporating smaller trim pots, and only the power on and clip leds.

[Cyber_Murphy]

Oh ok, well I have run in both stereo and bridged mono, I wouldn't want to lose that capability.

What I believe the Behringer does lack is a simpler faceplate, incorporating smaller trim pots, and only the power on and clip leds.

I agree about the faceplate too.. I was tempted to unplug and remove the LED display board and not worry about about having level display, and get a nice plain black or polished steel faceplate.. Perhaps even keep the disply board, but re-mount the single clip led from each channel through a new faceplate, so all you see is 1 clip led for each channel..??

Cheers..

[Cyber_Murphy]

Ok,, time for an update!!

I've completed the first stage of the mods, and that is to replace the stock op amps.

I replaced the 2 op amps from one channel to see how it would go.. Lucky I did only one channel,, because it hasn't worked LoL :o

The speaker pulsates and there is loud humming!! As you turn up the volume you can here the music getting louder but it's horribly distorted and the humming and pulsating get louder too!!

I imagine the LM4562 is not a straight "drop in replacement"..

The original mod uses OP2134 as "drop in replacements", but I couldn't get those and I figured using a newer better op amp would still be a compatible drop in replacement..

Maybe some component values in the surrounding circuit can be changed to make the LM4562 work, perhaps in the feedback circuit??

I have attached the circuit for the A500 input, and hopefully any Electronic Gurus out there might be able to see the cause (incompatibility) and know a solution,, which would be appreciated

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[Zaphod Beeblebrox]

You've discovered why techs are sometimes useful. The number one thing that techs DO NOT do, is to connect a pair of speakers to a (large) amplifier, after repair/modification. Damage is a real possibility. It is likely that your amp is oscillating at supersonic frequencies. Damage to the amp and/or your speakers is possible. Either restore your amp to it's original state, or send it to a tech who knows what they're doing.

[Cyber_Murphy]

You've discovered why techs are sometimes useful. The number one thing that techs DO NOT do, is to connect a pair of speakers to a (large) amplifier, after repair/modification. Damage is a real possibility. It is likely that your amp is oscillating at supersonic frequencies. Damage to the amp and/or your speakers is possible. Either restore your amp to it's original state, or send it to a tech who knows what they're doing.

Thanks ZB..

Yes, I am a tech of sorts, just not an expert with compaptibility between op amps...

The speakers I am using are my old junk speakers I always use for testing, not my actual speakers..

This mod is a well known mod for the A500,, its's just that I'm using a different chip to the one specified..

Got any (proper) technical advice on what could be happening?? What's so different between OP2134 and LM4562 that one is drop in and the other not??

Cheers..

[Zaphod Beeblebrox]

The ONLY sensible answer that can be provided is from putting your amp on a CRO. My technical advice has already been given. I suspect that your amp is oscillating at supersonic frequencies.

[Cyber_Murphy]

putting your amp on a CRO. I suspect that your amp is oscillating at supersonic frequencies.

Thanks ZB,

When i get home tonight I'll pull out the old sig generator & cro and have a better look.. I'll report back..

Although, I'm now just tempted to bypass the entire input section (removing both op amps and C53) and run it as true power amp (like i do on most of my amps with very good success),, see below..

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**EDIT: On further study of the circuit I've realised that the overall feedback for this amp comes from pin 5 of IC2, so this op amp can't be bypassed!!

Cheers..

Edited October 28, 2010 by Cyber_Murphy
Correction in my theory!

[Cyber_Murphy]

Ok, Time for another update..

Unfortunately the LM4562 is just not compatable with this circuit.. I'ts too fast and has too much bandwidth to work in the A500!

I looked at maybe changing DC offset, input current Bias, feedback loops, ETC.. This proved too complicated,, so I just decided to use BB opa2134's..

These WERE a "drop in" replacement and gave a nice result. Not a huge improvement to be honest, but a subtle 'nicer' sound, not as harsh, and offers an improved ability to pick out different instruments..

Also, when it came to by-passing the volume contol, I was actually able to go further and remove the entire input stage!! This meant removing the first op-amp and associated circuit completely, and injecting the signal straight into the input of the second op-amp, hence also bypassing all the 'nasty' capacitor coupling.. The now vacated TRS socket provided the perfect spot for a new RCA connector. Its bridge mode functionality is still available, you just need to use a shorted RCA plug on input 2, which shorts the opamp input to ground (which is exactly what usually happens when you have the volume at minumum for bridge mode)

The pic below shows the section of the circuit that's "no longer required". Shown is input 1, with 20K resistor across the input, which is usually how it looks with volume at max (R66 & R67 were never actually installed on the circuit board)

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This pics shows the empty input section, and the RCA socket installed on the rear chasis wired directly to the op amp input (via the 'former' volume pot connector)

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All in all I'm quite satisfied and enjoyed performing these mods!! I've only modded 1 of my A500's at this stage, so I plan to do A/B side by side testing (modedd versus stock) in the next couple of days in bridge mode.. I'll report back..

Cheers..

Murphy

[Paul Spencer]

Would be interested to hear about your little comparison.

When I first hooked up my A500 I didn't like the sound. It makes me wonder if there is something to this amp burn in talk. Then again, I tried it without the usual EQ settings, so that could be it. I never listen to my system without EQ as an integral part of the system. I've since had a chance to run it for quite some time and appreciate it's abilities. I find it to be fairly decent.

One thing I can say about this amp is be careful about driving it into clipping. I saw someone drive some open baffle woofers too hard, the amp went into clipping and got caught in a nasty feedback loop. The drivers kept moving and repeating bass that wasn't on the CD. Very freaky to watch.

Keith, if you want to learn some soldering, it's not too difficult. Just a few simple techniques to learn and voila! I find my biggest issue is being too impatient to set things up carefully. Little tricks like setting up a clamp or a way to hold things properly make all the difference.

I'm not a fan of the Vegas style lighting on the front. It's all a bit cheesy. Useful however. I watched a movie last night and was surprised to find that in some dynamic passages, it ate up all of the power this thing could put out! That wouldn't seem too crazy in a full range setup, but it's running an active mid from 80 - 2.4k and there is no boost. So this is like running the mains full range with 300w or more! I really do need some more efficiency.

Murphy,

I wonder if you might have more fun building an amp from scratch. I had a chance to see/hear an ESP power amp recently. It was in a nicely built case with an acrylic front, chunky Conrad heatsinks on the sides and some aluminium plates for the rest. Nicely put together. I believe it cost around $500ish. When I saw it I was tempted to build one, but then I remind myself of my chip amp sitting in the workshop that used to work!

Happy soldering ...

[Cyber_Murphy]

Thanks for the feedaback Paul,

Yes perhaps down the track I might remove the front dispaly. Maybe just keep the power, protect, and clip LED's but use a nicer metal front plate and a better on/off switch..

I think it sounds a bit cleaner now, I'm feeding signal straight from my modded DCX to the pin 6 of the power amp stage, so I think bypassing all the crap on the input has helped?? I'm happy to bring it along for a side by side stock versus modded listening test (sound familiar?) and get someone else's opinion..

As for building my own amp,, I have buily quite a few amps, but they've been Kit amps.. Do you mean "from scratch" to actually design the circuit, including board layout, making PCB Etc.. I'm not much of an electronic engineer,, I dont think I could easily design an amp from scratch

Cheers..

Murphy

[Paul Spencer]

After I said that, I saw you've already built one - that's what I meant by "from scratch." What I like about the A500 is that it's cheaper than you can build, it works, it's fairly powerful and it has a warranty! Funny thing is, I liked the look when I first saw it in a store, now that I have it, I'd much rather something more minimal. I think I'll just hide it in a rack, but you never know, I might get inspired to do some mods as well at some point.

[JA]

After I said that, I saw you've already built one - that's what I meant by "from scratch." What I like about the A500 is that it's cheaper than you can build, it works, it's fairly powerful and it has a warranty! Funny thing is, I liked the look when I first saw it in a store, now that I have it, I'd much rather something more minimal. I think I'll just hide it in a rack, but you never know, I might get inspired to do some mods as well at some point.

Hello Paul,

You could always remove the front panel and replace it with a simple brushed (IT MUST BE THICK !!!) aluminium faceplate and possibly even forget (remove) the volume pots or position them inside the unit to provide some gain setting as required.

Best

JA

[Paul Spencer]

JA, don't tempt me! I should check I'm outside of warranty first. I have too many other audio projects ...

[JA]

JA, don't tempt me! I should check I'm outside of warranty first. I have too many other audio projects ...

Here you go..... you can see all the supports and screws:

http://nice.kaze.com/a500_05_big.jpg

Amazing build for the $$$

Being a fellow minimlaist the only other thing I would do is to trim off the rack mounting lugs so that they were flush/underflush with the heatsink so that the new THICK ALUMINIUM front panel would be the same width as the heatsinks.

Best

JA

Edited November 10, 2010 by JA

[Cyber_Murphy]

Hello Paul,

You could always remove the front panel and replace it with a simple brushed (IT MUST BE THICK !!!) aluminium faceplate and possibly even forget (remove) the volume pots or position them inside the unit to provide some gain setting as required.

Best

JA

If you leave the pots at maximum, and you never intend to reduce them from max, removing them is probably a better option.. It saves having the signal having to travel right across the amp through a low grade volume pot, then back again.. If you look at the circuit I attached, and only use RCA, you can bypass even more unnecissary "signal interraction".. Either way, I just do it because I enjoy tinkering around, not really to gain huge improvements i suppose.. But I do seem to get little improvements when I do these sort of mods,, that's why it's good to have someone else's listenening opinion..

Cheers..