Amp Deal of the Century!

billbillw

maximillian wrote: »

Thank you again for this write up. I have made the changes you mention but can't try them out until I get home tonight. One thing I noticed is that I think you mean C505 and C506 instead of C503 and C504.

The only additional thing that I did is put a Vishay EMI filter cap on the on/off PCB (which I am placing in the same location as you but on standoffs).

I got the idea here:
http://www.10audio.com/diy_power_conditioner.htm

I will write back on how it sounds.

Thanks for catching my error. I went back and edited that post and made a couple slight changes.

maximillian

Helllo Bill,

I made the changes you outlined except I didn't change the volume pot yet. I still get a buzz sound that can change when I touch the pots bare metal shaft. Even if I put the plastic knob on I can sometime get a noise change when I touch the edge front plastic bezel. I can even touch the bare shaft the tone controls and get the same effect. Will changing the potentiometer eliminate this? Do you think the Vishay cap is adding noise? It seems to me that the tone control board's ground is very poor.

I appreciate the help.

billbillw

maximillian wrote: »

Helllo Bill,

I made the changes you outlined except I didn't change the volume pot yet. I still get a buzz sound that can change when I touch the pots bare metal shaft. Even if I put the plastic knob on I can sometime get a noise change when I touch the edge front plastic bezel. I can even touch the bare shaft the tone controls and get the same effect. Will changing the potentiometer eliminate this? Do you think the Vishay cap is adding noise? It seems to me that the tone control board's ground is very poor.

I appreciate the help.

That's because they cut the grounds on all of the control pots. If you look at the back, you can see this. It was like an after thought. Apparently the front panel is somewhat conductive too. Anyway, I I noticed the same thing, but it went away when I put everything back together. Make sure your power wires are all kept in that back corner.

maximillian

billbillw wrote: »

That's because they cut the grounds on all of the control pots. If you look at the back, you can see this. It was like an after thought. Apparently the front panel is somewhat conductive too. Anyway, I I noticed the same thing, but it went away when I put everything back together. Make sure your power wires are all kept in that back corner.

There is something really weird with the balance pot. I don't have time right now to investigate but two things I have noticed...
1) There is 68 Ohms between one of the channels and the pot's ground lug.
2) I have excessive noise in one of the channels, same one as in #1. If I unscrew the shaft's nut then the noise goes away. It seems like the front panel is conductive noise from one pot to the other.

I will try to look into it tomorrow night.

BTW, I removed the filter cap since it seemed worse with it in.

schwarcw

Kudos to Ben, Bill , Max, Fred, Madmax and man others who have contributed the 217 posts to this thread.

We could use someone ambitious to summaerize these into a DIY book with pictures, where to buy parts, various options, etc. I think you could eith offer this book for a nominal price to Polk members, or sell it ti the DIY /forum guys.

I think there's a couple of bucks to be made here.

Me, I have two of these little amps but I'm just waiting for someone to hit a home run with a DIY design. I think you guys are getting close.:)

ben62670

Working on it. I just received some more to play with. But really the total front panel bypass is the simplest mod that yields the best results. Fred's is probably the best SQ mod so far. Thanks guys guys. For the most part I have just been regurgitating info from here, and DIYaudio.
Ben

seeclear

seeclear wrote: »

anyone have any use for the removed board? I can send it to a fellow polkie if they have any use for it.

Anyone have any use for the removed board, or should I just chuck it?:eek:

maximillian

I made some additional mods and the unit is pretty much quiet now. I have started with Bill's mods. In addition I have done the following simple mods:

1) Insulate the potentiometers from the front panel. I cut the alignment stub off the potentiometers then applied insulating tape (I used Kapton) to the shaft hole in the panels for the volume and balance pots.

2) Shorted the volume potentiometer's ground terminal to the signal ground. It's cut on the PCB. I tried doing the same to the balance pot but this killed one of the channels (the same one with 68 Ohms).

With these two mods the amp is very quiet. Over three feet away I cannot hear any buzz/hum.

The last thing that pretty much eliminated the buzz/hum is to earth ground the heat sink/rear metal panel. I noticed that if I applied a finger to either the signal ground or the heat sink the little amount of noise vanished. The best solution I have for filtering this out at this point is by grounding the heatsink. I don't know where this noise is coming yet. It could be the crappy +/-15 supply but I don't have access to a scope to tell.

Grounding the heatsink is not the easiest thing to do. I know that a power inlet adapter can be bought that has the earth ground on the connector. I am going to find one and put a simple screw/lockwasher on the rear metal panel and jumper it to the inlet adapter panel. An alternate is to cut into the power cable and splice into the earth conductor.

Will keep you posted.

maximillian

I forgot to mention a couple things...
1) I did not change the pot as per Bill's mods.

2) The metal panel is connected to the audio signal's ground. So tying this to earth ground means the audio signal's ground (say from an Ipod) is tied to earth ground. So your source's signal ground would be tied to earth ground. Is this a "bad" thing to do? I think it may have the potential to create a ground loop. Thoughts? I will still look into better filtering instead of earth ground connection.

schwarcw

There seems like some really fundamental things that can be done to make this little amp sound much better. The bypass on the front end tone controls with the potentiometer seems to be the big back for the buck. Is the Rat Shack the best or does Parts Express have a smoother, quieter one. Isolating the potentiometer also seems to reduce some noise. The transformer also seems to provide some hum. Rotating the unit it goes away seems to be the solution. I also see some cap change out are also a potential for some improvement. the front end tube preamp also is intriguing. What about some changes to the power supply? By-pass caps? Better components. All these suggestions have been made under the guise of making some inexpensive modifications to a bargain unit and making it sound better.

My point was, may some of you guys can organize these experiences from the always brilliant members of Club Polk with some of the DIY forum enthusiasts. A short ten page manual with pictures and step bu step directions may become a popular publication.

Just my meager $0.02.

good luck!

maximillian

Finished the ground mod. I changed the inlet connector to one that was the same size for the panel hole but had the ground connection. I also tied two filter caps between both the line and neutral and ground (similar to the one that is there now). I am not sure if they are really needed. Then I drilled a hole in the rear panel and tied the ground wire here with a star washer and ring terminal.

The unit is really quiet. I am calling it quits for now. The noise that occurs when you touch the front panel or the exposed shaft of any of the controls is almost eliminated too.

Finally, I switched the left-right connectors for the input and output connectors to the pre-amp board to fix the reversed channel problem.

There is still some hum/buzz that is loudest in the middle position of the volume knob. It is almost unnoticeable under 2 feet.

The hum/buzz completely goes away when I put my finger on the input signal connector solder terminals on the preamp board. I think it has to do with signal ground still being noisy. I think cleaning up the +/-15V may be a good next step, but for now I am going to enjoy what I have.

IamKai

maximillian wrote: »

...I think cleaning up the +/-15V may be a good next step, but for now I am going to enjoy what I have.

Okay, but how did the mods make it sound to you?
What kind of difference did you get out of it?

maximillian

The best method to remove the hum/buzz is to completely bypass the tone control PCB as mentioned early in this thread. However, you lose all sound adjustments and have to rely on another method. Also, you really need a better pre-amp circuit to interface to the amp section. Some people hookup to a PC sound card which is powerful enough to drive the amp section. Hooking up the amp directly to an mp3 player will probably not be adequate.

Someone early on made a tube pre-amp mod that works real well. Billbillw's mods (and some of my variations) utilizes some of the tone control circuit with the hope of keeping the hum/buzz down.

I think it sounds very good for the price. You can see my and billbillw's posts, as well as the other mods, for sound quality impressions. It's a decent way to spend $15-$20. Of course, one could also find a used stereo integrated amp/receiver on Ebay for $50-$70 shipped. So this amp isn't the best and only solution. However, it was cheap and fun to play with. It also makes a great bedroom/garage system.

schwarcw

Nice writeup Max! I've got this amp sitting in a box for about a year now. I just have too many projects going on, but this is one that I would like to complete. Sound like fun!

Carl

billbillw

Yeah, lets be clear, this little amp is not going to replace your NAD integrated, your Rotel separates, or whatever you have for a main rig.

Its a cheap little amp that is built with pretty decent parts, and it does a very nice job amplifying a set of efficient speakers in a non-critical application. Perfect uses are for a computer rig, a patio/porch rig, bedroom rig, garage rig.

Consider this a lower cost alternative to the T-amp.

I use my 1st one for a computer rig because it is somewhat compact and matched with a pair of Athena AS-B1 bookshelves, driven by an Entech NumberCruncher DAC, it sounds very nice; much, much better than my previous Klipsch ProMedia system driven by a Soundblaster.

I plan to use my other one for the screened in porch running off my Zone2 output from the main system receiver. Outside on a deck is the sort of place you might risk having something stolen, or ruined by the weather, but being a $15 amp, it won't bother me too much if that happened.

madmax

One thing I have found out is that these amps are not the end-all solution to your sound endeavers. Recently I hooked a pair up to my Duo speaker setup. These speakers are 104 db efficient and have enough quality to show up an amp for what it is. Although I have heard better amps on these speakers I was pretty impressed that an amp I bought for $15 is adequate quality for this setup. We are into some pretty high class expensive amps here that start to make the Accurians look a little bad though. If for some reason I HAD to use the Accurian I would not be that depressed. Of course if your speakers are not that efficent then you can do better just because you are getting more power. Its been a long time now and I still have to give the Accurian amp its due...
madmax

MillerLiteScott

I did some rather crude mods to my RS amp. One of which was to mount it on a bread board to separate the power switch and the toroidal. I am running it as a straight amp directly from a variable output CDP. I have no hum or hiss with my ear right next to the tweeter. For some reason the switch does not work ( problem ). It comes on when you plug it in. Other than that it works fine as a straight amp. Looks like hell. I may build a new chassis for it out of some ipe mahogany, we shall see.

Scott

ben62670

I ordered some LM3886 chips, and I am going to do away with the whole circuit board. I will take two accurian amps and make two mono's out of the LM chips.
Ben

raudi

Picked up 2 of these a few weeks ago. I had the Kauai store ship them to my local store. I set one up the other day on my comp. Works great for a $15 amp.

fredv

Today I made a little change to the tube pre section as I felt the low was somewhat sluggish than I would like. I added a 2.2ohm resistor to the heater supply as it was too high - due to the transformer has much higher capacity than the tube draws. This should allow the tube to live as long as the amp can last :-) Next I replaced the 0.1uF/630V output caps (which was low but was the first pair that I found in the parts bin...) with a pair of parallel 2x1uf/63V Wima's. Hooked it up with the same source and speaker, powered it, yeah, that's what it should be!! For the sake of comparison, I hooked up a stock Accurian, played the same material, and shut it down in 10 mins :-)

MillerLiteScott

Fred,

Do you have a schematic of your mods? I have mine all apart and monted on a bread board and would love to put in a tube pre section. Do you use the original volume controls?

Scott

maximillian

Check out page 4 of this thread. There is also pics. Fred, do you have a complete parts list? Your photos show a brown transformer that doesn't appear in your schematic.

MillerLiteScott

Thanks,

That is way over my head. I think I will just buy one of the S5 tube amp kits.

fredv

Hi maxmillian, the little brown transformer is to power the tube's heater. That's the only parts is not on the schematic. It is Triad xformer
The only parts that is changed from the schematic is the output cap - from 0.1uF to 1uF or above. Due to the low B+, you probably can find quite a few of these from old electronics. Even bought new, the 4 caps that I put it cost me $1.5, am I cheap :-)

Hi MillerLiteScott,
An Accurian is an Accurian, no matter what you do :-) I have no doubt the S5 kit will surpass the performance. Yeah, the first time to touch these thing is kind of hairy, but once you complete one project such as a kit, your confidence will grow. Since you have whole amp outboarded, there is a lot of room to work with. You can do it!!!

BTW, if anyone has a hard time to source the 6GM8 tube but has some old 6DJ8/6922 tubes around, drop them in!!! I was surprised how low B+ the 6DJ8/6922 could tolerate. Now I can do tube rolling in this guy :-)

billbillw

ben62670 wrote: »

I ordered some LM3886 chips, and I am going to do away with the whole circuit board. I will take two accurian amps and make two mono's out of the LM chips.
Ben

You gonna breadboard the LM chips or use something from one of the premade kit vendors? I can't believe how cheap those kits are. You'll have to let us know how that turns out.

MillerLiteScott

After receiving some help from Both BillBill and Fredv I eliminated the pre-board and added both a source selector switch and volume control. I just finished mocking it up on the bread board to see if it would work and to my surprise it works. It is all but silent even at full volume, I have just a slight and I mean slight 60 hz buzz if you put your ear next to the tweeter at full power. I am not getting enough volume, IMO, the original pre-board must have increased the signal because I remember the amp getting louder in the stock configuration. It is loud enough but just nothing to write home about. This has been a great learning experience for me and I am looking forward to solving the volume issue and building a nice chassis for it. I am really looking forward to building another amp, probably a tube kit type amp in the Fall. Have a look at some of the picture I shot where it is powering my RTA 8TL's in my garage with an iPod as the source. It is still Fugly but it works.

Scott

ben62670

Tube pre next Scott. You'll get the gain, and then some that you need.
Ben

billbillw

MillerLiteScott wrote: »

After receiving some help from Both BillBill and Fredv I eliminated the pre-board and added both a source selector switch and volume control. I just finished mocking it up on the bread board to see if it would work and to my surprise it works. It is all but silent even at full volume, I have just a slight and I mean slight 60 hz buzz if you put your ear next to the tweeter at full power. I am not getting enough volume, IMO, the original pre-board must have increased the signal because I remember the amp getting louder in the stock configuration. It is loud enough but just nothing to write home about. This has been a great learning experience for me and I am looking forward to solving the volume issue and building a nice chassis for it. I am really looking forward to building another amp, probably a tube kit type amp in the Fall. Have a look at some of the picture I shot where it is powering my RTA 8TL's in my garage with an iPod as the source. It is still Fugly but it works.

Scott

Yes, the stock preamp board had two sets of opamps to increase gain. IMO, it needs them. Putting a volume control inline increases the resistance and lowers your gain even further. If you are just using an ipod, you can get by with using the Ipod volume alone. Otherwise, you need some gain in the circuit. I still think the mod I did is a good overall compromise.

Better opamp in the circuit, better volume control, re-route the power switch/lines, bypass the entire tone control section. It has plenty of gain and is very quiet.

MillerLiteScott

Bill,

Does the tube pre-amp mod you did increase the gain? If so that is my next project.

Scott

billbillw

MillerLiteScott wrote: »

Bill,

Does the tube pre-amp mod you did increase the gain? If so that is my next project.

Scott

That wasn't me, that was FredV.