Cheap Charlie amp solution for those power hungry AR's

[onplane]

My plan is to talk about a number of controversial issues in this thread and as such, some folks might feel slighted.  So my disclaimer is: I mean no disrespect to anyone.  My opinions are strictly that ... my opinions!

So now that I've repaired my TSW's (and they sound just fine), my amp is beginning to show it's age.  I power the TSW's with an HH Scott integrated amp (150 wpc 8ohms) that I purchased new back in 1987 (29 years ago).  Problem is the protection circuit comes on periodically for 2 seconds and then switches off returning power to the speakers.  It's been doing this for about a year now, but seems to be happening more often these days.  Given the age of the unit, I'm NOT going to devote any energy in tracking down the problem.  (Looks like the protection circuit senses heat and then opens the primary side of the main power transformer. Heat, as we know, can be caused by a whole host of things.)

So what to do??  Well, I've naturally been reading threads here about what other folks are doing.  I see many people are fooling around with tube amps!  That is NOT for me.  I assembled an Eico ST 70 many, many, many years ago.  I'm never going back to tubes.  Heck, I've got heat problems now and those darn tube amps run hotter than hell! The heat in those things will eventually fry ... every component! Solid state amps, on the other hand, run cool compared to tube amps.  Further, due to maintenance costs, plus the high cost of parts, there is just no way a tube amp will be an economical solution.  I will admit that there is one "nice" feature in a good tube amp and that is, the good units have multiple taps on the output transformers so powering a 4 ohm load should be easy!  Nevertheless, my AR4x's (all I could afford at that time) still sounded much, much better when I "upgraded" to an Eico solid state amp.

So tubes are out and that means I still need a solution.  In looking online at receivers, the vast, vast majority are home theater units packed with lots of little amps, none of which could begin to handle my AR TSW's or my AR-3a's.  I found by qualifying my search by adding "stereo" I did locate a number of modern receivers with decent power output (100 wpc) at decent prices (under $200). However, in checking further, I didn't find any of these modest priced units that are rated for 4 ohm loads.  When you begin to require 4 ohm capability, you can find units, but prices become astronomical! 

 

Then by accident I stumbled upon this Sherwood unit (see attached datasheet).  This unit is rated at 100wpc into 4 ohms full frequency (this is NOT one of those little home theater amps rated at 1000 Hz).  Further, it has 4 such amps!!!  That's right not two (left and right channels), but 4 (L,R + L,R).  

So how does this help us drive those power hungry AR's?

Well, you've got a full 100 wpc at 4 ohms from the first amp and that will satisfy most.  For some of the really power hungry AR's you can now bi-amp using a single box!!  That is, your woofers are powered using the A speaker outputs.  The mids & tweeters are powered using the C speaker outputs.  You have separate source, volume and tone controls for each of the two amps.

Now for the really controversial part, with this unit you can safely bi-amp your AR-3a's using the original 3 terminals.  So instead of using pots to adjust the "voice" of the AR's, you dial in the voice you want by simply varying the volume controls on the two independent amps.  Further, you can vary the voice with the remote control to suit your desires on different sources or different music. Remember if you are playing old vinyl, you want to return to the 3a's original voice. At the same time, they are just too dull on modern digital playback devices.This is a really neat solution and the delivered price to me is $160.

Splitting the load on these power hungry AR's (via bi-amping) makes all kinds of sense and means you can economically drive these power hungry units without having to mortgage the house. One final thought is with this scheme you gain some headroom, because you have a full 100 wpc for just the woofers and then another full 100 wpc (which you'll never, ever need) for the mids and tweeters.

Regards,
Jerry


Sherwood-RX-5502-datasheet.pdf

[ar_pro]

Without plunging deeply into the archives, isn't this more or less the same sort of thing that you were promoting like ten years ago?

[onplane]

1 hour ago, ar_pro said:

Without plunging deeply into the archives, isn't this more or less the same sort of thing that you were promoting like ten years ago?



 

Well, sort of.  Yes, I have been bi-amping my 3a's for the last eight years.  But ... but I never had an elegant solution like this unit.

For those AR's that are factory setup for bi-amping, this unit is a "drop in".

In my case, using the original three terminals and amps with dissimilar power, I had many problems. Such as:

1. Can we really hook the ground terminals on different amps together (that is, are both amps really common ground)? This is dangerous if they are NOT. However, I've never seen a solid state amp that isn't common ground, but there might be some out there.

2. How do you get the same signal to the 2nd amp? My solution was to select one amp as the master and the other as the slave. The master controlled the source and the record out is fed to the aux input on the slave.

3. How do you insure with dissimilar amps that both halves of the speaker get driven in phase? I mean for the most part this is 50 - 50.  Fortunately, I can verify with my dual trace scope. 

All of these problems go away with this 4 amp unit in a single box! I mean you can run 3 lines from the amp to the 3 terminals on the 3a's and start playing with the volume controls to set how they sound.

One very interesting test would be to wire one 3a in the standard single amp configuration and the other 3a in the 3 wire bi-amp. Then set the volume on both amps to exactly the same level and see if you can hear any difference. So what we'd have is exactly the same voltage going to all parts of both 3a's so in theory, there should be no difference. I predict, however, there will be a difference, because each amp will "see" a different impedance. When you split the networks apart the impedance of each half is better behaved. That is, fewer peaks and valleys.

 

One more digression, on the phase issue that I found particularly troublesome. The first woofer amp I used was a Heathkit with a "flat" switch which would override the tone controls. Turns out that switch also reversed the phase which I could clearly see on the scope. So by pushing a switch I could immediately change the phase of the signal going to the woofers. Naturally, I couldn't resist the temptation to compare music with the woofers both in and out of phase with the mids and tweeters. 

And as I often do, I listened to 3 tracks and found better resolution and separation with the drivers out of phase. Naturally, I posted that "terrific find" here. Well, that turned out to be permature! After I posted, I kept doing A/B comparisons and things went horribly wrong.  On other tracks I would lose separation and definition when the drivers were out of phase! Finally, I then ran into a whole bunch of tracks where I couldn't hear any difference whatsoever! This is NOT neat and my only conclusion is that phase probably gets "mixed up" when the audio engineers assemble tracks into the final product. In any event, since then my current amps only allow for in phase and I not messing with it any more, because this stuff will drive you .... crazy!

 

 

[onplane]

Just thought I'd "close the loop" on the "Cheap Charlie" amp solution. 

I did purchase and replace that old Scott amp with a brand new Sherwood RX-5502 a little over a week ago. This Sherwood receiver is NOT a Home Theater unit nor is it a simple Stereo unit. It's a little unusual in design. It has a stereo amp rated full frequency at 70 wpc @ 8 ohms or 100 wpc @ 4 ohms.  Naturally my interest was in the 4 ohm capability and this unit is fully capable and will drive my AR's well beyond my comfort level. (It is also capable of powering two sets of 8 ohm speakers in parallel.)

So what I described so far is a standard stereo receiver designed to handle 4 ohm loads, which in itself is a good thing. That's not the end of the story, because in this box there is an additional amplifier rated at 100 wpc @ 4 ohm. In short, this is a terrific solution for those who would like to bi-amp their AR's. In particular, it's a terrific solution for those who would like to bi-amp vintage AR's with only three terminals.  

Reason I say this is because with this unit you can run three wires to each vintage AR and "split the load" over two independent amplifiers. Now you do end up with two volume controls, one for each half, but since volume controls are digital you can insure each half is getting exactly the same voltage.  Naturally, you can vary power between halves when you desire to change the AR's voice.  Reasons for altering "voice" range from room acoustics, to source, to personal preference. AR recognized this and that's why they had those pots.

Unfortunately, even with the pots "full on" you still have base heavy, "dull" speakers. Varying power allows you to voice any way you want. Next, splitting the load (off loading the mids and tweeters from the woofer amp) is also a good thing.

Enough about bi-amping, so let me share my impressions of Sherwood's most unusual unit. First off it's bigger than a bread box! By that I mean it's physically bigger and heavier than most stereo receivers on the market today. Most of the weight is due to the power transformer. The output devices' heat sink is thick, but it looks like aluminum. Cooling is done by convection. That is, there are no annoying fans and the unit's feet lift the unit up over a half inch to help create a "draft".

As for controls it's really a "no frills" unit. Via front panel or remote you can control independently each amp's volume, source and speakers. Naturally, you can also control the FM section, but I haven't played much with that.

There is no "loudness" control, but there is a "tone direct" (aka tone bypass). So you could set tone controls for low volume and then bypass when you turn the volume up. 

Just a couple of surprises, unit has a phono preamp and phono input.  Warrantee period is a full two years.

For $160 delivered, so far I am totally satisfied.

My one area of concern (and I have it will all units that have remote controls) and that is with the voltage regulators.  They provide the low voltages for the digital circuits and these darn things run so hot, they eventually cause problems. The only way I know how to minimize is to turn the units full OFF.  That is, don't leave them in standby for prolonged periods.

Hope you find this interesting...

Jerry

 

 

[stan461]

Very interesting. Thank you for sharing. No need to apologize for an economical solution. My request is that you post your impressions after using the new setup for a while. 

I periodically swap in speakers, amps and preamps into my system. One of the units I use is a Marantz SR7000 as a preamp. It has a remote which comes in handy to adjust the volume differences when streaming youtube for music thru an Apple TV. Anyway, the Marantz is always in standby when not in use and never gets close to hot. Is the Marantz better designed? What am I missing here?

[onplane]

10 hours ago, stan461 said:

Very interesting. Thank you for sharing. No need to apologize for an economical solution. My request is that you post your impressions after using the new setup for a while. 

I periodically swap in speakers, amps and preamps into my system. One of the units I use is a Marantz SR7000 as a preamp. It has a remote which comes in handy to adjust the volume differences when streaming youtube for music thru an Apple TV. Anyway, the Marantz is always in standby when not in use and never gets close to hot. Is the Marantz better designed? What am I missing here?

Stan, you are right. I also have a Marantz AV receiver and it runs cold in standby mode.

Actually I left the Sherwood in standby the last two days and it's pretty cool as well.  Looks like the voltage regulators do a lot more work when the amp is on. Besides the digital circuits, I'm guessing they provide the B+ and B- for the preamps.

What's really a mystery is after driving my AR's for over 5 hours the heat sinks for the output devices are cool! All of the heat (and there isn't much) comes from these two voltage regulators.  Conversely, the Kenwood amp I use on just the woofers for my 3a's gets warm after an hour. 

Now, the Sherwood's output devices have to be dissipating power so how is it possible there is no heat? I guess if you design a box to hold 4 100 wpc amps, efficiently getting rid of the heat becomes a top priority. The Sherwood unit must create a very good air flow over the large heat sink.

 

Regards, 

Jerry

[genek]

Unlikely that there's no heat, unless you're consuming so little power that the mass of the chassis is enough to dissipate it. If so, you'd have to open the case and actually touch the heat sinks to notice anything.

My main system is an older Sherwood 5.1 pre and separate power amp. Rated 170wpc down to 2 ohms, at normal volumes I can feel a bit of warmth in the case but nothing comes out of the vents. I think the front panel lights may actually generate more heat than the amp stages.

[onplane]

Since my last post, I felt that I really should "push" this "cheap charlie" amp to see whether it will really deliver the superior performance that the unit appears capable of providing. I know what you are thinking, “Oh boy, another one of Jerry's stories where everything went well ... UNTIL ....!”

Fortunately, by sheer luck there is no typical "until disaster struck" in this story.

For the past two days, I have been driving my TSW's with all 4 amps within the Sherwood. I guess I'm a "control freak", but I just love it!

OK, so you are probably wondering how I use 4 amps on two speakers each with just two terminals. Well, I removed the 12 inch woofer and brought another line out (when I say "line", I mean a "cheap charlie" line of speaker wire through the back of the cabinet - nothing fancy here!). In the drawing below, you can see that this line goes to the coil in the woofer xover. Notice that now I have three terminals, just like my AR-3a's!  Also notice that to return to a single amp driving a speaker, all I do is twist that "new" wire together with the lead from the amp and insert them together into the Red terminal. (I physically confirmed that this works.)

Naturally, the big change is using an amp for the woofers and another amp for the mids/tweeters. After I made one TSW resemble an AR-3a terminal configuration, I hooked up the two amps to it. I then set the volume in both amps at exactly the same digital setting and compared the sound to the other TSW being driven by a single amp. Theoretically, there should be no difference in sound since all halves of both speakers see exactly the same signal at the same voltage.

Well ... I did hear a minor difference, but it's very hard to characterize. It seemed that the speaker with two amps was just a tad clearer. That is, I could pick out the instruments just a tad better. Now, was this "perceived difference" due to the two vs single amp? Was it possibly due to room acoustics, minor differences in the speakers, or minor differences in the amps? Who knows!

A more interesting question is: should we expect any difference in distortion caused by the amps in these two different configurations? Here the theory is clear. As amplifiers work harder (i.e. put out more power) distortion increases. Indeed, your amp's power specification doesn't mean that your amp won't put out more power. It means that beyond that specified point, distortion is unacceptable. When we look at the amp powering the mids/tweeters it's power output (as defined by P= I x V) is 60 to 70 percent lower than the single amp driving the full speaker. Thus, we should expect lower distortion in that signal. Now, is that theoretical lower distortion really audible? Well ... who knows?  My point is everything is going in the right direction when you "split the load".

So, would I go through all of this trouble just to get this minor improvement in clarity? No way!  Heck no! It's not worth it! Far too much work for the slight improvement, but that's NOT the end of the story. What you do gain is control over voice and this is a big deal!  You gain control over voice by means of complete control over the power going to each half of the speaker. This is NOT the same as tone controls. Treble tone controls cut or add typically in the 10K to 15K range while the bass control works in the 50 to 100 Hz range. 

Using volume controls you effectively change speaker voice very similar to the pots on the 3a's. Except volume controls are far more powerful than pots PLUS they are on the remote control so you can change on the fly to your personal preference. When I say "more powerful" I mean there are limits to the voice you can achieve with pots. At full ON, 3a's are still bass heavy. Conversely, my 3a’s are what I want them to be!

Now my TSW's have no pots and by comparison, they are a "bright", "in your face" speaker. Early in the day, that is exactly what I want! Later at night, when reading or working, I find I turn the mids/tweeters down and revert to the classic AR bass heavy sound. That is, I'm quite comfortable with the AR woofers thumping and growling away. Late at night I just love hearing the bass player going up and down the scales and AR's reproduce this just so much better than any bass reflex system.

 

 

Anyhow, back to the "cheap charlie" Sherwood amp.  It has performed flawessly. Being able to alter voice with a few remote clicks is really, really, really nice. Now is it the "perfect" bi-amp solution? No, it is NOT the perfect solution! For one thing, when bi-amping you should gain "head-room". I'm fairly confident you gain no head-room at all with the Sherwood unit. Reason I say that is it's my guess that all amps within this box have a single power supply. Thus any rail "droop" or "jitter" will impact ALL amps.

Another feature lacking in the Sherwood is separate tone controls for the individual amps. For example, I like to turn the treble control full off for the woofer amp and the bass control full off for the mid/tweeter amp. The idea is by reducing the frequency bandwidth we hope to cut some intermodulation distortion.

What the Sherwood has going for it in a big way is cost!  For $160 delivered, I don't believe there has ever been a bi-amp solution anywhere near this price range. Further, it works just fine with just three terminals.

Hope you find this interesting ...

Regards,

Jerry