Bob Cordell's Power amplifier book

[PB2]

Hello Pete

I mean at home when you listen to your hifi what amp do you use in your system. I am assuming you like the sound of it.

Regards
Arthur

Hi Arthur,

I am do for an upgrade, but generally, my system sounds so good that I am not compelled to replace anything and have not been for many years. My amps are not any sort of benchmark, they are good enough for now and I am planning to build something new as time permits.

Have you been to any of Bob's workshops? I was able to reliably pick out the tube amp in his tube vs. SS A/B test and while you might think that they would be drastically different, they were not. Many "audiophiles" chose the SS in this blind comparison.
This is the one I attended, HE 2007 about half way down the page:
http://www.stereophile.com/taxonomy/term-a/215?page=3

 

[Bob Cordell]

Well that is certainly an interesting topology, lots of transistors and boosted front end supply rails all adding to cost and complexity. This is probably a good time to ask the question, Bob, do you really think you (or some of the best trained listeners) could pick out one of these advanced topologies as compared to some of the better but much simpler ones in a blind A/B test?

As I've mentioned before, my brother and I were both highly interested in amplifier design decades ago and this is where we ended up. Sure it is challenging to design out as many non-linearities as possible but there are only so many hours in the day and one should ask is this the best use for our time - might our time be better spent designing better loudspeakers for example. It should reach a point where the question arises - where is the threshold of audibility?

If this is a relaxing passtime for some then sure go ahead and spend as much time as you like - but the question is certainly valid.

I am aware of Geddes' work on the audibilty of distortion - yes I know it is complex.

Hi Pete,

The question of whether one could distinguish among these different topologies is always a good one. I listen a lot, but have done virually no blind testing. I have done occasional A/B listing tests, such as the kind we did at HE2007, but I honestly don't hear the dramatic differences that some claim to hear. This doesn't mean those differences don't exist. There are much better listeners out there than me.

I DO believe that a well-engineered amplifier costing a couple thousand dollars can sound as good or better than some costing 10's of thousands. Not all of the really expensive ones are as well engineered as some of the ones described on this site.

I have never gone into an amplifier and chaged resistors to see if I could hear the difference. I stick with good-quality metal-film resistors, sometimes well over-rated in power dissipation, and that is it.

For me this is indeed a relaxing pastime and as an engineer I enjoy the challenge of making a better amplifier. That does not just mean going for the lowest THD, but also going for the best, most civil behavior under all operating conditions and speaker loads. Good clipping behavior and high output current capabilities and non-intrusive protection are important examples.

Boosted IPS-VAS supplies, in my opinion, are a good performance enhancer for the cost. I agree, nothing is for free and we do not live in a world where cost is no object. But how one prioritizes the spending of substantial amounts of money counts. There are many good ways I'd spend the money before I'd pay thousands of dollars for exotic speaker cables and the like.

As a person who has designed many DIY loudspeakers, both active and passive, I totally agree that different loudspeakers make a much bigger difference than two well-designed power amplifiers. Check out my patented EQSS low-frequency loundspeaker technique on my web site.

Cheers,
Bob

 

[Zero D]

Boosted IPS-VAS supplies

Agreed

For the little extra it costs it can make a BIG difference, in Many ways. Naturally everything else needs to be designed etc carefully, but that's a given.

I'm sure a lot of people would be very pleased with the results, even if they modded "ordinary" Amp circuits. Try it & be impressed

 

[Bob Cordell]

Just thought I'd mention Bob that I used your device models in a simulation of the old Universal Tiger amplifier and found some rather odd behavior. I expected to see cross conduction, but most likely less with the faster NJL outputs as compared to the MJL outputs. Instead the NJLs showed cross conduction whereas the MJLs did not. Also, the PNP MJL had a rather high saturation voltage of about 4.5 Volts at 10A peak, I expected to see closer to 2 Volts. Just looked up the spec and it is 1.4 V Vce Sat at 8A. The PNP part does have more beta droop but it seems to be more than it should be.
I do appreciate all the work that you did and that you made these available but I was wondering if you noticed this issue.

Here's a link to the simulation files in case you are interested:
http://www.diyaudio.com/forums/soli...-tiger-improved-simulation-2.html#post2583843

I dug into it a bit more and it might be caused by the higher beta droop of the PNP device, I was using a 4 ohm load and it does better into 8 ohms.
I don't usually see this in real life, and one reason might be that the worst case beta of 25 at 8A is at TJ of 25 deg C, the outputs get hot when driven hard like this and beta goes up with temp so typically this amp would go within about 2 volts of the rail even with a 4 ohm load.

Hi Pete,

Thanks for bringing this to my attention. The higher cross-conduction for the faster device is hard to understand. I don't recall the model numbers right now, but could it be higher Ccb capacitance in the NJL devices? Strange. I assume you've biased them apples-to-apples. You might want to look at the behavior of the driver collector currents in both cases.

I'll take a look at the MJL21193 model and see if it looks way off on sat. Sat is not one of the characteristics I was working hard to get accurate, but that seems pretty far off. I also note, however, that my experience was that it was really hard to get the very high current behaviors of the transistors right without sacrificing accuracy in some way at lower currents. Base spreading, beta droop and ft droop are really tough to get right in the 5-10A neighborhood.

Cheers,
Bob

 

[Bob Cordell]

New transistor models available

New material has been added to my website and I've finally gotten many new transistor models posted. I welcome you to have a look at the site and let me know if you find any errors.

The model additions and revisions took me much longer than I thought they would, and I apologize for it taking so long to get them up onto the site. For most transistors I've tried to incorporate values of base resistance that give a reasonable correlation with actual measured transistor noise results. RB is another area in which manufacturer's models are simply awful. It seems like most of the time they just punt.

In fairness, however, it does involve compromises to choose a value of RB that will be reasonable for both DC results (e.g., Vbe vs. Ic) and AC results, like noise. Base spreading resistance behavior makes life difficult in this regard. For small-signal devices, I've generally erred on the side of trying to get the noise right in the current regions where the transistors are typically used.

Cheers,
Bob

 

[PB2]

Thanks again Bob for all your hard work on the models.

I looked at the MJL simulation again and interestingly enough, if I raise the load impedance to 16 ohms I do see cross conduction, probably because this allows for a lower Vce that gets the devices futher into saturation. I would expect these to be able to provide more current than the NJLs but perhaps they need more base drive to do it and get deeper into saturation. Reading the spec again, they require .8A of base drive to do 8A Ic with 1.4 Vce, and 3.2A of base drive at 16A Ic and 4 Vce. The 100 ohm resistor to ground in the emitter lead of the driver in that amp limits the max driver Ic to about .4A, this should probably be reduced to 50 ohms if one wanted to use the MJLs and have strong drive into low impedances. Clearly the MJLs do not have much gain at 16A, but in real life hot they probably would do much better. I should probably point out the the higher Vsat that I'm seeing is with a 4 ohm load and a peak Ic of about 9.8A so perhaps the sim is just an absolute worst case.

I hope, at some point, to get some MJLs and try them in that amp to see what they actually do.

 

[PB2]

I noticed that you have the 2n5087 paired with the 2n5089, when actually the 2n5210 is the comp version since they are both 50V and about the same beta. The 2n5089 is a 25V part with higher beta. No big deal of course ...

Also wondering which BD139/140 you modelled, there are the -10 and -16 versions - I assume the base version?

And to nit pick, I see that you're use the C suffix to indicate that they are your models, but there are the C beta selected versions of parts like the BC550 - a bit confusing but probably hard to change now. If you did do the C version of the 550 would it then be CC? I suppose that is reasonable.

 

[PB2]

Hi Pete,
As a person who has designed many DIY loudspeakers, both active and passive, I totally agree that different loudspeakers make a much bigger difference than two well-designed power amplifiers. Check out my patented EQSS low-frequency loundspeaker technique on my web site.

Cheers,
Bob

Were you on the Bass e-mail discussion list years ago, roughly the mid 1990s? I often pointed out how low tuned vented systems behaved more like sealed, in fact in the limit as the tuning frequency goes to zero they are the same. There is a general model that is often used in software simulators where if you make the passive radiator compliance infinite, it becomes a vented model, and if the port mass is infinite a sealed box model.
I commented on tuning the NHT1259 in a vented or PR system to 20 or even 16 Hz and many on the bass list just did not understand the idea or were resistant to it. There are many who hate vented systems and just do not understand the excursion advantages. I finally built a system with 1259s tuned to 20 Hz and they are mighty impressive - a pair on each side.

Port turbulance and compression can become a real issue, there was an excellent article in the AES by Gander about it years ago. It is well known, but I don't think many realize how strong the effect can be - Gander put some measurement based numbers on it.

 

[Bob Cordell]

Were you on the Bass e-mail discussion list years ago, roughly the mid 1990s? I often pointed out how low tuned vented systems behaved more like sealed, in fact in the limit as the tuning frequency goes to zero they are the same. There is a general model that is often used in software simulators where if you make the passive radiator compliance infinite, it becomes a vented model, and if the port mass is infinite a sealed box model.
I commented on tuning the NHT1259 in a vented or PR system to 20 or even 16 Hz and many on the bass list just did not understand the idea or were resistant to it. There are many who hate vented systems and just do not understand the excursion advantages. I finally built a system with 1259s tuned to 20 Hz and they are mighty impressive - a pair on each side.

Port turbulance and compression can become a real issue, there was an excellent article in the AES by Gander about it years ago. It is well known, but I don't think many realize how strong the effect can be - Gander put some measurement based numbers on it.

Hi Pete,

You're certainly right about the imporatnce of port turbulence. In my subwoofer I use a 4-inch port flared at both ends. I think it is about 15 inches long.

I used the EQSS technique in my 3.5-way Athena active loudspeakers. People could not believe the low-end I was getting out of them, especially given that each channel used only four 5-1/4 inch woofers (I wanted to keep the cabinets narrow).

Cheers,
Bob

 

[Bob Cordell]

Hi Arthur,

I am do for an upgrade, but generally, my system sounds so good that I am not compelled to replace anything and have not been for many years. My amps are not any sort of benchmark, they are good enough for now and I am planning to build something new as time permits.

Have you been to any of Bob's workshops? I was able to reliably pick out the tube amp in his tube vs. SS A/B test and while you might think that they would be drastically different, they were not. Many "audiophiles" chose the SS in this blind comparison.
This is the one I attended, HE 2007 about half way down the page:
HE 2007 | Stereophile.com

Hi Pete,

I should mention that the tube amplifier used in the workshop A/B listening tests will be the subject of a design article in the next issue of Linear Audio. I, too, was surprized at how many people mistook the tube amp for the big Denon 200+ wpc amp it was being compared with. A complete report on the workshop is available on my web site at CordellAudio.com - Home.

Cheers,
Bob

 

[CBS240]

Hi Pete,

I usually like to use boosted rails for the IPS-VAS in my amplifiers that strive for highest performance. There are a couple of reasons for this, but simply using boosted supplies will not automatically result in better sound. Its what they let you do to improve the circuits.

For me, the key is that the boosted supplies provide more headroom for the whole input section so that there are more design choices and fewer constraints in making a high-performance IPS-VAS. I first used boosted rails in my MOSFET power amplifier with EC because I was using a VAS with cascodes, etc. I never wanted the VAS to come even close to a region of voltage headroom discomfort. In that amplifier, the boosted rails also helped with the fact that the MOSFETs needed more turn-on voltage than BJTs.

If you don't use boosted rails, you may waste precious headroom in the main high-current power supply. In the overall scheme of things, those extra volts from a low-current boosting supply are quite inexpensive.

The use of boosted rails also give you the opportunity to really do a good job filtering the supplies to the IPS-VAS, more effectively keeping power supply noise, ripple, etc away from those more sensitive parts of the amplifier.

It is undoubtedly true that the addition of boosted rails will add to the cost of the amplifier, but if we are talking about really high performance amplifiers it is but one of many more higher costs.

Hi

I too have discovered the advantages of boosted rails for the IPS and VAS. It is almost required for vertical mosfet SF OPS. I did simplify the idea with my current amp (+/-24V), I use a simple half wave voltage doubler followed by a small signal series-shunt voltage regulator. Since only ~15mA is required there is not much wasted energy in dropping 10V across the regulator circuit and end up with +/-38V for the IPS and VAS. This allows me to use the same transformer leads as the main high current supply and achieve plenty of head room to drive rail to rail output.

 

[jcx]

driving rail-to-rail on output has the downside of running the output devices into deep saturation where their gain, time constants change rapidly giving possible poor loop stability, bad clipping recovery

 

[Bonsai]

driving rail-to-rail on output has the downside of running the output devices into deep saturation where their gain, time constants change rapidly giving possible poor loop stability, bad clipping recovery

Yes I agree, but you have to clamp to prevent this. If your VAS stage incorporates current limiting (easy on both single ended or balanced types) then a straight crude diode clamp on the VAS output to the output stage (lower) supply rails will do the trick. The important thing I believe it to prevent the OPS from saturating, and especially so on bipolar designs because of the time it takes to suck out the base charge.There are more sophisticated methods, and Bob discusses these in his book.

 

[Bonsai]

"Many "audiophiles" chose the SS in this blind comparison."

Hell, now there's a surprise!

 

[Zero D]

rail-to-rail

@ jcx

Yes , nasty. But as long as the input signal is kept below driving the output into clipping, it will be fine. These days a lot of pro amps have inbuilt limiters, which if designed, & then set correctly, should never clip the amp.

@ Bonsai

You offer some good suggestions.

 

[Bob Cordell]

driving rail-to-rail on output has the downside of running the output devices into deep saturation where their gain, time constants change rapidly giving possible poor loop stability, bad clipping recovery

This is exactly right, and a situation that should be avoided in amplifiers using boosted rails for the IPS and VAS. Baker clamps are an obvious choice. That is one reason I implemented Baker clamps in my MOSFET power amplifier with error correction. Note that if you have an output Triple EF, you can choose to clamp the output of the VAS to the main rail voltage (or a filtered version thereof) and largely keep the output transistors out of sat. All sorts of diode arrangements are available to allow you to choose how close you want to get.

Allowable gate swing for power MOSFETs needs to be considered as well. One has to make the choice as to how far to forward the gates, recognizing that one is getting into high Cgd territory.

Cheers,
Bob

 

[ostripper]

This is exactly right, and a situation that should be avoided in amplifiers using boosted rails for the IPS and VAS. Baker clamps are an obvious choice. That is one reason I implemented Baker clamps in my MOSFET power amplifier with error correction. Note that if you have an output Triple EF, you can choose to clamp the output of the VAS to the main rail voltage (or a filtered version thereof) and largely keep the output transistors out of sat. All sorts of diode arrangements are available to allow you to choose how close you want to get.

Allowable gate swing for power MOSFETs needs to be considered as well. One has to make the choice as to how far to forward the gates, recognizing that one is getting into high Cgd territory.

Cheers,
Bob

Good timing , for my final version of the "DBT" , I want the clamps. In (figure 10.7 - P193) , what would you hook Vp and Vn to ? The rails ? Would you be clipping across the diodes , not the OPS devices ??

OS

 

[Bob Cordell]

Good timing , for my final version of the "DBT" , I want the clamps. In (figure 10.7 - P193) , what would you hook Vp and Vn to ? The rails ? Would you be clipping across the diodes , not the OPS devices ??

OS

In the figure as shown, I'd connect them to a filtered version of the 45V rail that is at least 2V below the 45V rail in order to give the current source at least 1.3V to work with. This is probably a good example of where at least a small boosted rail would be helpful. With a boosted rail, I might connect the diodes to a filtered version of the main rail.

Cheers,
Bob

 

[LineSource]

The discussion on VAS clamp diodes reminded me of Glen's K12A design. Glen combined the diode clamp with his solution for setting a fixed current on complementary differential LTPs with both cascode and current mirrors.

Has anyone built Glen's K12A who can comment on sound quality, and how well his VAS clamp+current_bias solution works?

 

[Bob Cordell]

The discussion on VAS clamp diodes reminded me of Glen's K12A design. Glen combined the diode clamp with his solution for setting a fixed current on complementary differential LTPs with both cascode and current mirrors.

Has anyone built Glen's K12A who can comment on sound quality, and how well his VAS clamp+current_bias solution works?

Hi Linesource,

One of the most complex 12W amplifiers I have erver seen .

Anyway, I took a quick look at it and I couldn't find the clamp diodes, but it is early in the morning.

Cheers,
Bob