Internal signs of a solid built amp?

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Internal signs of a solid built amp?

Postby Member02 » Sat Jan 07, 2006 10:43 am

I used to know most of them but upon looking inside an old DEI 500 4 channel amp I realized my numbers in my head were off.

So I'm just hoping for anyone and everyone with some knowledge about the different parts inside the amp to add soem input on what makes a good amp. and if anyone knows how to get any info out of the caps and transformers, etc...
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Postby eezip » Sat Jan 14, 2006 1:09 pm

Oh man, I'm glad you asked. I could type for days on here about what I think makes a good amp. But you'd be [at least I am] on how good performance can be obtained from 'cheap' parts. Honestly, a lot [if not, perhaps, most] of an amp's performance has to do with the PCB layout but the parts used to tell a story. Here are a couple things I am thinking right now:

1. If you see ICs by Fairchild, TI, Philips, National, IXYS, APT, STMicro, etc. then the manufacturer paid money to get good parts from a known manufacturer. Same with caps; Nichicon, ELNA, EVOX/Rifa, Wima, Xicon, and Rubycon are just a few of the good brands of capacitors. Capacitor value and voltage is printed on the cap if it's an electrolytic. Most smaller thru hole caps also have that information. SMT aluminum electrolytic and tantalum caps tell you what they are as well, but chip capacitors normally have no marking. As for transformers, that's a lot harder. Since you usually can't trust the core color to give you the material for sure, you just need to get the power out, estimate an output rail voltage(s), know input voltages, guess [or determine] a switching freq and then decide if the wire guage and constructin seem adequate for the job. If a transformer can output 1/3 RMS output power continually then you're probably in good shape. If it can only support 1/8 output power then you're looking at an undersized supply. But that doesn't preclude good performance.

2. See where stuff is. Does the signal flow from input processing, to the amplifier input, to the output make logical sense? Does the power get processed and meet the amp right at the output stage and then go directly to the output? Those are all good things. If the different parts [input, amp, power supply] are thrown in willy nilly and cross over each other all the time then you run the risk of signal contamination and you also may be dealing with a designer who didn't plan well before he started working on the amp. Obviously, that's probably not the best plan. If wires are flopping around all over the place I get the impression the designer could have planned things out better and saved hand placement of parts.

3. If you're familiar with electronics, bore into the amp and see what exactly is in there. If a class D amp designed last week has multiple paralleled output transistors I start to wonder why. Perhaps it's for economies of scale but better performance can probably be obtained using a single larger device, with many other benefits. In my opinion, less is more. Getting the maximum functionality out of a circuit with the last parts is a real art, and fewer parts means longer reliability. Check out some of the parts if you are able to compare them and see if the amp is using good ICs.

There are a million things. If you have a specific picture of an amp then post it, ask questions, and I will give you my thoughts on it. Everybody has their own opinion and there are plenty of amps out there with great reputations and are designed vastly differently so not every amp designer agrees.
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Postby mikechec9 » Wed Jan 25, 2006 11:51 pm

here is a pretty good link:
http://caraudiomag.com/specialfeatures/0111cae_anatomy/

also eezip, if you don't mind, could you take any of these http://ampguts.realmofexcursion.com/ and perhaps compare a couple of pics just for clarity's sake.

this might also make a good reference: http://ampguts.realmofexcursion.com/Eclipse_DA7232/inside1.jpg
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Postby eezip » Tue Jan 31, 2006 12:51 am

mikechec9,

Thanks for the links. The first one, Anatomy of a Power Amp, is rife with misinformation. In the second paragraph the author puts that a lot of the ancillary protection circuits can be controlled by a micro. That's just not true. The tide is beginning the change but the vast majority of protection circuits are either discrete analog or built into an IC.

Then when discussing power supplies the author implies that regulated supplies have less ripple than unregulated designs. He may mean that regulared supplies will not drop the rail voltage under load, but ripple is the AC component riding on a DC voltage and either type of supply can have great or crappy ripple charateristics.

In the amplifier section the author writes that the distortion happening when the output transistors cross zero voltage and 'hand off' the load to one another is 'notch distortion' when it is usually called crossover distortion. And they claim that class A amps require less feedback to attain the same distortion levels as push-pull amps which is again not true. The amount of feedback needed to achieve a certain THD performance at a given operating point depends on a whole lot more than the output stage design. That's way too broad of a statement.

He goes on to correlate the parts of the driver stage with the term Darlington which may or may not be the case. Darlington is a specific arrangement of transistors and you can get great results with and without Darlington-arranged transistors. He makes is seem like Darlington arrangements are mandatory.

Now we get to the really grotesque statement: "The more output transistors, the better. Multiple output devices reduce distortion (requiring less negative feedback) and improve reliability." This is absolutely, without question blatent misinformation. Although looking at a lot of amps you might think this is true, in fact the number of output devices an amp uses is between the designer's judgement/feelings and economies of scale. If your company can get a certain transistor, which is cheap but wimpy, and nothing bigger, you may have no choice but to parallel multiple transistors. Furthermore, more parts always means less reliability. A well-designed amp with lots of parts may have a very long life even under abuse, while an amp with low parts count might fail right away. It depends on the design of the amp and the odds of getting a part that just fails earlier than normal because of some manufacturing or soldering/assembly problem.

The bipolar vs. MOSFET debate again isn't correct. Both varieties of transistors came in varying power levels so it's really a design choice. One doesn't hold a clear advantage over the other as far as I know, although some amplifier designs or markets may be more suited to one type over the other.

This article can't be terribly recent because for quite a few years now high quality full-range class D amps have existed. In the conclusion the author equates different amplifier classes to different listeners or types of music. That's a blanket statement because the class of amp doesn't necessarily have anything to do with it's use. Although it's true that the bigger the amp, the more power can be saved with a higher efficiency design like D or T, they certainly aren't all "dirty" and solely suited for LF reproduction.

So while I am attacking many parts of the article a lot of it I agree with. There are some good points made that I am a big champion of such as: the D in class D doesn't mean digital. That drives me crazy. Anyway, you wanted a little dissertation on some amps guts from the pictures. Ok, next post...
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Postby eezip » Tue Jan 31, 2006 2:06 am

Let's start with amps I have personally taken apart and know at least a little bit.

1. Kicker DX700: http://ampguts.realmofexcursion.com/Kicker_DX700/inside1.jpg
The first thing I notice is that it uses the HIP4080 full bridge gate driver/modulator. While it does have a couple quirks that are unfortunately hard to squash in audio use, it's an excellent gate driver for motor control and does well in audio, especially if you don't need to reproduce 10kHz or higher. The bridge is comprised of beefy Fairchild FETs grouped tightly together on the top right-hand side. There are secondary storage caps right by the FETs. Yay! The output filter is pretty tight around the FETs and the gate driver/modulator is spaced about equally from each FET. It would be nice to have the gate driver/modulator a bit closer to the FETs but the compromises made seem ok to me, especially for when this amp was designed. The input section wraps around the right and lower side of the board and doesn't cross over the power supply rail traces or the amplifier output section. Also a good thing.

The power supply looks like a push-pull design what with an equal number of power devices spaced equally from a large number of windings on the primary side. Also, the pri caps split up next to the trio of devices leads me to think it's a push-pull. Not to mention the fact that the vast majority of car amps use a push-pull supply.

Having the amplifier, input section, and power supply split up as they are not only helps to reduce parasitic coupling but also can help create a simple and very effective grounding scheme. All in all, the DX700 is quite impressive. Considering the state of semiconductors and class D in general when it was released, I'd give it an A-/B+.

2. Tsunami dB1100: http://ampguts.realmofexcursion.com/Tsunami_db1100/inside1.jpg
The power supply is another typical push-pull design. It uses an unfortunately large number of power devices but should still work ok. The input filter is quite burly and it has loads of promary capacitance. The RLC combo in the upper right corner is a trap, which will cut down the carrier of the class D amp and substantially improve THD if done right. It's right at the output and looks good. Other than that, the amp is a bloody mess. It uses a pair of power supplies that appear of feed a pair of half bridge amps which are permanently bridged together. The amplifier uses both P-channel and N-channel FETs which is never a good choice and is particularly dissapointing. The seconrdary caps are located relatively close to the output transistors, which is good, but the modulator and drivers are located a long way from the FETs and the grounding doesn't look particularly well-thought out. The input section has no ground plane and while that's not intrinsically detrimental a ground plane rarely hurts at audio frequencies. The magentics look to be low-quality the trap inductor uses a steel screw right through the core material which I imagine wasn't taken into account when the trap was designed. Furthermore, while a relay can provide reliable protection, solid state protection can be designed with far more functionality and grace. With the proliferation of quality gate drivers when this amp was designed I can't see why you wouldn't take advantage of them, except for cost. The modulator is potted so I can't see it but it looks like a couple of SO8 ICs are there so I an guessing there is a RC timer creating the carrier wave. The amp is just big and very inelegant from my viewpoint. The RE 12.1 appears to be a very similar design but with less power supply transistors [haven't I been harping about that?] and a different gate drive scheme.

3. JBL BP1200.1: http://ampguts.realmofexcursion.com/JBL_BP1200.1/inside1.jpg
I haven't actually seen this amp in person so I will have to go solely off the picture here. As with the dB1100 the amp has bajillions of transistors. Why don't some of these companies pony up cash for some newer, higher current devices so there aren't so many of them paralleled. Less devices means greater reliability and often more PCB layout options which can be a key to good performance. It looks like the power supply secondary caps are staggered both at the power supply output and around the output devices. Good. You can see a symmetical drive scheme spreading up and down from the center of the amp to the output transistors. Unfortunately, the switched output appears to go thru the floating wire to the output inductor right to the middle of the PCB! Yikes! Then the two halves are filtered and the amplifier output crosses over half the power supply to get to the output terminals. That's something I would avoid. So not too bad but if more thought was given as to where things should go before laying the board out it might have been even better.

Let me say that every designer has preferences and things I would do don't necessarily guarantee good performance. And there could very well be constraints that I can't fathom which dictated the design. Nonetheless, I think there is a certain dignity in knowing that by starting over from the beginning when you're 50% done with a design you will end up with a far better product. Nothing helps a PCB layout get better than doing it once, seeing what is screwed up, and then doing it over again. I don't know where these amps are made but in the USA single-sided PCBs are virtually nonexistant while in China single-sided PCBs still enjoy a significant cost savings. It's things like these that drive commercial designs while DIYers who are making a couple for themselves and friends scratch their heads and say "what the....".

Again, in no way am I well qualified to critique these designs from the original designer's perspective but this is what I am seeing from a relatively quick look at a couple high power class D sub amps. I hope maybe I provided some insight into what one person thinks when investigating a design. Please let me know if [or if not] a big post like this is helpful and/or appreciated. If you got all the way through thanks for reading and please disregard my probably poor grammar :)
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Postby westend » Sat Feb 04, 2006 6:58 am

Thanks Eezip for your opinions on amp design. Priceless. I have two four channel amps. One is a Soundstream Rubicon and the other, a Planet Audio. The PA amp uses 1/3 the devices as the SS amp. The layout is very linear and well thought out. It was built by Steve Mantz, who now operates Zed Audio. If anyone wishes a more involved knowledge of amp design you can read through Steve's technical papers he has listed here: http://www.zedaudio.com/techtalk.html
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Postby eezip » Sun Feb 05, 2006 7:22 pm

I read all the tech papers and while I didn't check his math [although it looked all correct at first glance], I have some quibbles about a few points. Here we go in order of listing:

Amp Classes:
Pure Class B amps have no crossover distortion because the transistors turn on and off at exactly the same time.

Amplifier Clipping:
He is using 1.414 as a multipler for crest factor [RMS to peak value]. That's only true of sine waves - typical music has crest factor of 3 or more! He also isn't mentioning the fall off of music power as frequency increases which drastically lowers the chances of clipping high frequencies.

Amplifier Efficiency:
He writes "Class D are PWM amplifiers and have no relationship with analog designs." This is absurd - class D amp are completely analog. The copyright says 2005 but even serveral years ago there were good full range class D amp available. Steve should update this document with newer info.

Amplifier Protection:
He makes it sound like VI limiting is bad but VI limiters are handy and don't intrinsically mean high harmonics or bad performance.

Balanced Line:
The 'inverter' circuit he uses is silly. Using 2 less resistors an equal amplitude balanced transmitter can be made. Also, using 500ohm output impedance is ridiculous. Typical line level outputs would be 100ohm or less which lessens any voltage [signal] drops.

Cables:
I can't agree more. Only super cheap cables make a difference for audio gear, and the difference even then is slight. Sadly, the document doesn't appear to be finished. Here he correctly states the 10-100ohm [or so] output impedance of most line level gear.

Capacitor Types:
Thank God! Large value, quality electrolytics don't intrinsically sound any worse than film types. If THD+N of an amplifier is below 0.005% with a bipolar electrolytic, you can't hear it! And that bipolar electrolytic will cost only a few pennies in quantity. I'd like to back Steve up in saying that, in my opinion, C0G/NP0 ceramics are great for most all audio use.

Components:
I do wish this document was finished as I agree with a lot of what Steve said in the introduction. Audible memory is terribly short and I place a low importance on listening tests compared what it seems like many others feel. Only once I have good performance on the bench do I listen to something. And I strongly believe that you can measure what you can't hear - meaning that if I can hear something it can most certainly be measured. Unfortuantely, it is sometimes quite hard to figure out exactly what in the circuit is causing whatever it is you might be hearing. On the other hand, I think that I can measure things that you can't possibly hear.

I feel he supplied a bunch of good information which everyone interested in the inner working of amplifiers and their applications should read. Unfortunately, he peppered it with poor grammar and incorrect lower case units. Furthermore, lots of statements appear to be propoganda making Zed products out to be superior to other companies. I feel he is making a big deal out of some very trivial things which either don't matter or are in common use in many amplifiers, but from the papers it appears Zed should have a billion patents for all these great ideas. Steve has been around for a while and certainly knows his stuff so if you can get a handle on what he's provided here you'll be better off for it.
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Postby jlaine » Sun Feb 05, 2006 10:23 pm

eezip wrote:I read all the tech papers and while I didn't check his math [although it looked all correct at first glance], I have some quibbles about a few points. Here we go in order of listing:

Amp Classes:
Pure Class B amps have no crossover distortion because the transistors turn on and off at exactly the same time.

Amplifier Clipping:
He is using 1.414 as a multipler for crest factor [RMS to peak value]. That's only true of sine waves - typical music has crest factor of 3 or more! He also isn't mentioning the fall off of music power as frequency increases which drastically lowers the chances of clipping high frequencies.

Amplifier Efficiency:
He writes "Class D are PWM amplifiers and have no relationship with analog designs." This is absurd - class D amp are completely analog. The copyright says 2005 but even serveral years ago there were good full range class D amp available. Steve should update this document with newer info.

Amplifier Protection:
He makes it sound like VI limiting is bad but VI limiters are handy and don't intrinsically mean high harmonics or bad performance.

Balanced Line:
The 'inverter' circuit he uses is silly. Using 2 less resistors an equal amplitude balanced transmitter can be made. Also, using 500ohm output impedance is ridiculous. Typical line level outputs would be 100ohm or less which lessens any voltage [signal] drops.

Cables:
I can't agree more. Only super cheap cables make a difference for audio gear, and the difference even then is slight. Sadly, the document doesn't appear to be finished. Here he correctly states the 10-100ohm [or so] output impedance of most line level gear.

Capacitor Types:
Thank God! Large value, quality electrolytics don't intrinsically sound any worse than film types. If THD+N of an amplifier is below 0.005% with a bipolar electrolytic, you can't hear it! And that bipolar electrolytic will cost only a few pennies in quantity. I'd like to back Steve up in saying that, in my opinion, C0G/NP0 ceramics are great for most all audio use.

Components:
I do wish this document was finished as I agree with a lot of what Steve said in the introduction. Audible memory is terribly short and I place a low importance on listening tests compared what it seems like many others feel. Only once I have good performance on the bench do I listen to something. And I strongly believe that you can measure what you can't hear - meaning that if I can hear something it can most certainly be measured. Unfortuantely, it is sometimes quite hard to figure out exactly what in the circuit is causing whatever it is you might be hearing. On the other hand, I think that I can measure things that you can't possibly hear.

I feel he supplied a bunch of good information which everyone interested in the inner working of amplifiers and their applications should read. Unfortunately, he peppered it with poor grammar and incorrect lower case units. Furthermore, lots of statements appear to be propoganda making Zed products out to be superior to other companies. I feel he is making a big deal out of some very trivial things which either don't matter or are in common use in many amplifiers, but from the papers it appears Zed should have a billion patents for all these great ideas. Steve has been around for a while and certainly knows his stuff so if you can get a handle on what he's provided here you'll be better off for it.


You sir have a small plethora of knowledge in amplifiers... Thanks for being so helpful around here and sharing it like you do. :)
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Postby eezip » Mon Feb 06, 2006 1:41 am

Thank you Josh :) I am always hoping I'm useful and it's nice to hear someone confirm my hopes.
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Postby mikechec9 » Sat Feb 18, 2006 2:08 pm

Glad the links were of use. Your valuable input on the subject is appreciated. Now i just need to take the time to peruse:cwm12:

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