Amp Deal of the Century!

seeclear

I just finished modding my first one, and I'm listening to "Joy to the World" through Atrium 45's using my laptop to play the mp3 as the source! So cool. Anyway, a ? from this total noob--I noticed there was some hum before I plugged in the jack to my laptop audio out. Once I plugged in the jack, the hum was totally gone. Do I need to do anything like the "toroid turning" that I saw before to deal with the hum, or is it moot if it goes away when the source is connected? I would think it is no big deal, but I thought I would ask the experts, anyway. What say you?

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

dcmeigs

If you still want one (or another) of these, you can go to a rat shack company store- not a franchise store- and get one shipped to you house under their direct2u program. They pay the shipping. Mine just arrived. $15 plus tax shipped.

billbillw

dcmeigs wrote: »

If you still want one (or another) of these, you can go to a rat shack company store- not a franchise store- and get one shipped to you house under their direct2u program. They pay the shipping. Mine just arrived. $15 plus tax shipped.

Unless the store is right near you, the gas cost to go to a store might outweigh the shipping cost. I think I paid $7 for shipping two amps, purchased from the Radioshack website link that is in the first post of this thread.

dcmeigs

True. But you can't swing a cat around here with out hitting one.

What kind of gas hog are you driving anyway?

billbillw

dcmeigs wrote: »

True. But you can't swing a cat around here with out hitting one.

What kind of gas hog are you driving anyway?

I get an average of 26mpg in mixed city/hwy driving, not a Hog at all, BUT, many people are lucky to get 20mpg. Also, my car requires Premium, like many of the higher performing cars today. When I drove by the station today, Premium was over $4 per gallon.

Bottom line, if the store is more than 15 miles away, it going to be a wash for most people.

BTW,
I've completed some excellent mods to one of my amps. I'll have a write up short. Basically, I found a way to make this thing completely hum/buzz free, retain the volume/balance control, bypass all the crappy tone controls, and improve the sound greatly. Very, very little expense incurred. More later.

maximillian

billbillw wrote: »

BTW,
I've completed some excellent mods to one of my amps. I'll have a write up short. Basically, I found a way to make this thing completely hum/buzz free, retain the volume/balance control, bypass all the crappy tone controls, and improve the sound greatly. Very, very little expense incurred. More later.

That's excellent. I have been looking for a way to improve the SQ while maintaining the volume and balance. I don't care about the tone controls either. However, I think maintaining the gain of this circuit is probably a good idea so the amp doesn't sound too weak. Does your mod retain this? Can't wait for the details.

billbillw

maximillian wrote: »

That's excellent. I have been looking for a way to improve the SQ while maintaining the volume and balance. I don't care about the tone controls either. However, I think maintaining the gain of this circuit is probably a good idea so the amp doesn't sound too weak. Does your mod retain this? Can't wait for the details.

Yes, I kept one of the op-amps in the circuit but disabled the second one that is in the tone circuit.

Ok, here is what I did.

1st, I relocated the power switch to the left top rear corner (see 1st and 2nd picture). Just unscrew it from the tone board and remove the button. I glued the button back into the front panel to maintain the look.

I had to drill a small (approx 3/8") in the top side panel, then I used a wire nut as a button. Use whatever you have for that. I drilled a 2nd smaller hole in the pillar and re-used one of the mounting screws to secure it back there. Getting the AC feed away from the tone board gets rid of any sign of buzz that I previously had.

Next, I did the following mods to the board.

I removed C505 and C506 and put small jumpers to replace.
I removed both opamps and replaced the one the left side with a socket and installed a Burr-Brown OPA2134 (you can get free sample from them). I removed the 2nd one so that it wouldn't steal any power from the very limited 15V feeds.

Next, I removed C509, C510, C523, and C524. These are the last caps before, after the tone circuit. I placed jumpers across them to bridge the circuit from the output of the Op-Amp directly to the trace that feeds the output plug. (Look at the 3rd photo for details on exactly where to replace the jumpers).

I also replaced the volume pot with a RadioShack Alps pot (only costs $3.50), but this is not pictured. The RS pot wires up exactly the same, but the pins don't fit, so you need need to have jumper wires for all 6 connections. Also, the RS shaft is slightly bigger (1/4"), so you need to drill out the hole to fit. I just used a 1/4" drill bit and did it by hand (clamped the bit into the vice and hand rotated the knob down onto the bit).

This is all I have done so far, but it seems to work great. The RS volume pot tracks much better at low volumes. There is no noticeable hum or buzz. There is quite a bit of noise at full volume, but what amp doesn't have that. Its essentially quiet in the first 1/2 of the volume range, which is where it will mostly be used.

Additional mods that I plan:

I will probably remove C501/C502 completely. These are coupling caps (to block DC) that are probably not really needed. The amp board already has a set of coupling caps which would prevent any DC from getting to the amp itself.

I will probably replace the coupling caps on the amp board with BlackGates if I have the right size laying around.

I will probably also add some decent size (330uF?) electrolytics bypass caps to each of the 15V feed lines to help stabilize that supply.

I could replace some of the resistors with better quality metal film resistors (which might lower the noise floor), but at this point, it might be a case of diminishing returns.

Right now, it seems to be a very worthy amp for $20 and an handful of very inexpensive mods.

ben62670

Thanks for the right up. I was getting ready to dive into mine. I should have one in on Tuesday.
Ben

dcmeigs

Great write up. Thanks.

I don't think I understand your motivation for moving the power switch. Was it necessary?

billbillw

dcmeigs wrote: »

Great write up. Thanks.

I don't think I understand your motivation for moving the power switch. Was it necessary?

Absolutely! The main AC power switch being right next to the preamp board was causing an audible 60hz buzz. Moving the switch eliminated that buzz completely.

Another option would be to somehow insulate the preamp board (or the AC lines/switch), but that seemed like it would be more work.

Having that switch and those main power lines running right next to the pre- board was a very poor design choice.

Some people don't notice the buzz, but in my use as an amp for near-field speakers (sitting less than 3 feet from each speaker), the buzz was very distracting.

maximillian

billbillw, do you know anyone who has traced out the schematics of the preamp board? If yes, do you know if they are available online? I am thinking of doing it but don't want to waste my time if someone already has.

billbillw

maximillian wrote: »

billbillw, do you know anyone who has traced out the schematics of the preamp board? If yes, do you know if they are available online? I am thinking of doing it but don't want to waste my time if someone already has.

Not that I know of. There is a complete parts list at the DIYaudio.com site, but no schematic.

http://www.diyaudio.com/forums/showthread.php?postid=1309162#post1309162

What are you trying to figure out? I've traced the board pretty well and can pretty much tell you what everything does on the first (volume) side of the board. The only thing that I don't know is the tone control section. I didn't bother with that because I planned to bypass it anyway.

maximillian

I just wanted to see if there is some other things that could be improved. For example, maybe there is a way to improve the tone controls.

billbillw

maximillian wrote: »

I just wanted to see if there is some other things that could be improved. For example, maybe there is a way to improve the tone controls.

Sure there is a way to improve the tone controls: Better pots, better caps, better resistors. But that is lots of work that is not worth it IMHO. I prefer flat response anyway and usually use the tone defeat, direct mode, or whatever its called to bypass the tone controls.

IamKai

Face wrote: »

I agree. After bypassing the tone board, if I use my PDA as a source, the amp is hardly loud at all.

Pardon the newB nature of this question please, but is it possible to delete a resistor somewhere to bring the sensitivity up on the amp after bypassing that opamp?

It would be very nice to have it able to be used as a standard power amplifier without any of the front panel enabled. I am trying to make a simple switch setup that would allow either use at the flip of a switch.

billbillw

IamKai wrote: »

Pardon the newB nature of this question please, but is it possible to delete a resistor somewhere to bring the sensitivity up on the amp after bypassing that opamp?

It would be very nice to have it able to be used as a standard power amplifier without any of the front panel enabled. I am trying to make a simple switch setup that would allow either use at the flip of a switch.

It will work fine with the preamp board totally bypassed, but you really need a preamp type source as opposed to an ipod/PDA type device. Those devices are designed for powering headphones, not an amp. Its an impedance mismatch.

IamKai

I just did the mod outlined in post 171.
OMG!

I lost sensitivity to be sure, but the increase in quality is just unbelievable!!

Having several of them, I was able to match the output (the setting of the volume control) on a unit with the tone board still enabled.

There is no contest!!! I am shocked beyond words at how GOOD the thing becomes when you do this mod to it.

I wish there was a way to retain the sensitivity and keep the quality.

I am going to try the tube input mod, but I wonder if there could be a little more in the way of pictures of it to guide newB's like me through the process.

Anyone who has a good preamp available and wants to hear what that board does to the sound, should do this mod. The difference is not a subtle one.

Also, shielding the input leads to and from the plugs helps a little bit.
(Am I the only one who shielded the input wires?)

IamKai

billbillw wrote: »

It will work fine with the preamp board totally bypassed, but you really need a preamp type source as opposed to an ipod/PDA type device. Those devices are designed for powering headphones, not an amp. Its an impedance mismatch.

I found it to be quite a bit less sensitive than my Crown D150A or my Audionics CC2.
I was able to get good power from the Accurian, but it took a lot more volume out of the preamp to do it.
(My pre is an Adcom GFP-565.)

Thanks for the info on this! Had it not been so easy, I may never have tried it...now I am aware what this little thing is capable of. It really is shocking.

billbillw

IamKai wrote: »

I just did the mod outlined in post 171.
OMG!

I wish there was a way to retain the sensitivity and keep the quality.

There is. If you don't want volume control at all, just do the mod that I outlined in post #190! Or, If you don't want volume control or balance, then jumper from the 1st pin (furthest left) of the volume pot for each channel to the input legs of the opamp. That should be pin 3 and pin 5. The only thing you need to worry about keeping in the loop are the feedback caps/resistors for the opamp (R505/506, C507/508), and the small blue cap(s) and resistor(s) that are at the input (R501/502, C503/504). You can bypass C501/502.

ben62670

IamKai wrote: »

I found it to be quite a bit less sensitive than my Crown D150A or my Audionics CC2.
I was able to get good power from the Accurian, but it took a lot more volume out of the preamp to do it.
(My pre is an Adcom GFP-565.)

Thanks for the info on this! Had it not been so easy, I may never have tried it...now I am aware what this little thing is capable of. It really is shocking.

Welcome to club Polk, and glad to see you are enjoying yourself. The tube mod bumps the input voltage up to around 2.5v, and needs to be attenuated. I have not done this yet. Also I wanted to do a headphone out directly from the tube. I don't know how it would sound, but its a near free mod.
Ben

DeadAccurian

Hey guys.

Found this forum and thread on Google. I just lost my Accurian Sender (15-1938) to a power surge. (When plugged in, I get nothing.)

When these things were being discontinued at RS, I bought one pair of powered speakers, three stand-alone receivers, and one (duh) sender. And now my one sender is toast.

So, anyone think it can be saved? Anyone have any ideas or tips on where I could go to have someone look at it? (There's a good TV repair shop right near me; was thinking of talking to them.)

Thanks for your input, folks.

(BTW, I'm the proud, proud owner of a pair of SDA loudspeakers.)

jw

ben62670

DeadAccurian wrote: »

Hey guys.

Found this forum and thread on Google. I just lost my Accurian Sender (15-1938) to a power surge. (When plugged in, I get nothing.)

When these things were being discontinued at RS, I bought one pair of powered speakers, three stand-alone receivers, and one (duh) sender. And now my one sender is toast.

So, anyone think it can be saved? Anyone have any ideas or tips on where I could go to have someone look at it? (There's a good TV repair shop right near me; was thinking of talking to them.)

Thanks for your input, folks.

(BTW, I'm the proud, proud owner of a pair of SDA loudspeakers.)

jw

First welcome to Club Polk.
Second have you opened up the device to see if any parts are obviously burnt? There are a lot of sensitive parts in the sending, and receiving boards in these devices. You might want to look for a sender on Ebay.
Ben

DeadAccurian

Hey, thanks, Ben.

Yep, opened it up looking for the obvious and all looks well. I've also looked on eBay and contacted one seller.

Do you think the parts inside are somewhat commonplace and could be replaced if neccessary? (Sorry, total noob question, but I'm not proud.)

jw

ben62670

The problem is there are a lot of surface mount chips on the sending board. They basically operate like TCP/IP realtek network cards(peal the label off one of the chips). The chips are fairly sensitive, and prone to failure with voltage spikes.

DeadAccurian

yeah, so I saw when I opened it up. Can those be replaced? Can someone with the right equipment find out which one?

(Again, sorry for the uninformed questions. Just kinda hoping I can resurrect this one and not have to try to find (and pay for) another one.

ben62670

DeadAccurian wrote: »

yeah, so I saw when I opened it up. Can those be replaced? Can someone with the right equipment find out which one?

(Again, sorry for the uninformed questions. Just kinda hoping I can resurrect this one and not have to try to find (and pay for) another one.

I think it would be hit or miss. Most likely miss, it will still cost ya, and I don't know how good the repair would hold up. MHO

IamKai

a tech friend of mine said that they usually had issues with the pwr supply and not the 'radio' portion of the device.

if you had a couple of test leads and a meter, you could do a preliminary check of the thing. It could be something really silly and simple.

the manager at one of the storess told me they do have the transmitters for the people who had the extended warranty on them. How true that was, I dunno.

IamKai

ben62670 wrote: »

Welcome to club Polk, and glad to see you are enjoying yourself. The tube mod bumps the input voltage up to around 2.5v, and needs to be attenuated. I have not done this yet. Also I wanted to do a headphone out directly from the tube. I don't know how it would sound, but its a near free mod.
Ben

If you plan on doing this modification, I sure would like to see pictures of it step by step.
It has intrigued me since I first saw it.

ben62670

IamKai wrote: »

If you plan on doing this modification, I sure would like to see pictures of it step by step.
It has intrigued me since I first saw it.

Its burried in this thread somewhere. Fredv has done it;)

maximillian

billbillw wrote: »

I removed C503 and C504 and put small jumpers to replace.
I removed both opamps and replaced the one the left side with a socket and installed a Burr-Brown OPA2134 (you can get free sample from them). I removed the 2nd one so that it wouldn't steal any power from the very limited 15V feeds.

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.