250w 8ohm amplifier

[a.wayne]

No, i won't count out the Superamp. I understand why you had those problems and it does not worry me as i am not going to have my boards etched. They are going to be cnc'd. I don't like the risk of corroding too much or too less. I'll just take a layout, draw it in autocad, which is fairly simple and send it to the cnc machine programmed to the exact dimensions. I'll only use the peak of the drill when cnc-ing it so that i can get the lines pretty close to each other if that's the case.

Also, i already talked to the guys at the electronics shop and they told me i can measure the transistors before buying so that's gonna be a big save if i am required to match them.

My questions about this amp:

1.Does it have protections? Which?
2.Output power in 8 ohms at +- 75v
3.How well should the transistors be matched?
4.Where the hell can i find the schematic and layout? (top, bottom, top and bottom)

The same questions apply to Apex B500 if anyone's willing to answer them...

Thank you!

Interesting i have never heard of a CNC. Board before, Why ..?

 

[a.wayne]

Here is the website for the Superamp. It has all the info you are asking for. Note that most of the construction info is on the Low TIM pages which are linked to in the Superamp docs. It would be good to read through both before starting. Dr Leach was a professor and used these amps for teaching purposes so there is a lot of really good info.

So far I have not been able to drive my to clipping. As I said earlier, once my dummy load arrives I will be able to test for that. I should be able to use my Variac and set my PS to +/-75V and give you some idea of what it will do.

On another note. I have been reading about the Studio reference amp That Apex designed. It is supposed to work on +/-75V. You may want to look at that.

Blessings, Terry

How much outputs can the low -tim amp be scaled up to without modifying the VAS ...?

 

[a.wayne]

Hi Guys

Considering that all of the Dx Blame variations are compromises of the original Blameless circuit, I would not bother with them. Certainly the fact there are board images available is enticing, but... I have not looked carefully at those to see what compromises have been made of the layout.

Again, I would strongly encourage you to look at the Leach Low TIM design and just expand it to the power you want. Cascoding is not needed to get more power TODAY. Just increase the supply values to +/-80V to +/-90V and use four pairs of output devices. Make sure all the active devices can tolerate the higher supply voltages.

Also note that just because your woofer is rated for 250W does not mean that it will tolerate that much power in the cabinet you are using. Speaker electrical ratings are for the voice coil only, and reflect use in a tight box with a lot of air resistance to cone movement. In a vented design, openback, or simply a larger sealed enclosure where the cone can move more freely, the amount of power required to reach full excursion is much reduced. This means the driver is effectively derated power-wise - maybe 100W pushes it to the excursion limit?

You say you've built some 50W or so amps. Do you have clip lights on these? Have you tried them with your intended cabinet load for the new high-power amp?

I think if you made some measurements to see how much power you use on average, the zeal for a lot of power might recede. On the other hand, it might show you that if you listen to dynamic music, that most of the time power is within that 50W window and just the peaks are being clipped. In that case, you should seriously consider making a class-G output stage.

In class-G, there are two supply levels, usually with the higher being twice the lower. The output stage looks superficially like a cascode but operates quite differently and does not have the plague of oscillations. This kind of amp is essentially identical to the Blameless up to the output stage. The same number of output BJTs is used as for the expanded Low TIM circuit, so eight devices per channel. You could build the Low TIM Leach, expanded to high voltage but with the class-G output.

The class-G output allows a reduction of the heat sink size and reduces the average waste heat from the amp. Idle conditions for the output stage are based on the lower supply rail value. The front-end and drivers must operate from the high rails for full output to be achieved.

Have fun
Kevin O'Connor

Here you go .....

I didn't get a chance to swap any transistors to day but I did get a chance to reset the bias. When I was done, I ran some tests using the sinewave generator and scope. I discovered that the generator will put out more voltage on a Square wave so I tested with with that. I still couldn't get the amp to clip. Here are some pics of the test.

Left channel, generator on left and op on right
http://smg.photobucket.com/user/sti.../IMG_20130627_145029_391_zps860a34d5.jpg[/IMG

left channel, generator on bottom, output on top


Right channel, generator on left, output on right.


Right channel, generator on bottom, output on top.

Great pics , what freq ..?

DX MK3 for kevin ...

 

[wg_ski]

The Low-TIM can be scaled up to about the same as the Superamp and operate off +/-84V. Use MJE15034/5 drivers and five pair of MJ51024/5. That puts its output capability and SOA the same as the 1974 version of the CS800, with the sound quality of the Leach. Higher rails require more heatsink on the VAS and because IdQ goes up, but the MJE340/350 are ok up to 20mA idle current in that service. Updated 120V input devices are available and there are so many to choose from these days.

The S-Leach topology came about because the MJ15025 simply wasn't available when the amp was designed. Something had to give on voltage to be able to use PNPs. The commercial standard CS800 used all NPNs, and they held up fine on that rail voltage with 5 pair.

 

[TonyTecson]

the latest leach lo-tim board group buy here was a 3 to 6 plastic pairs option ones...
instead of more plastic pairs, one can choose to use TO3 types for greater SOA...
as for VAS and other trannies, Greg Erskine made a tabulation for such here...VAS Transistors
you can see the others on this webpage...

250 watts into 8 ohms means 45 volts ac volts(sine) output...
minimum rails is about +/-70 volts at full power..
depending on the size of your power traffos, your unloaded rails will be much higher than +/-70....

 

[a.wayne]

I'm ok with the fact that this amp is grey, no coloration, because i can achieve that using an eq. Anyway, in my opinion, a colored amp should be chosen in relation to the speakers it's going to drive, otherwise it would sound odd.

What i am more interested in is if this blameless amp is able to be detailed. Now, what i don't know is what makes an amp be detailed. Or better, what makes an amp LESS detailed, which i think (although i am not sure) is the distortion. My theory is that once it distorts, it distorts all frequencies, therefore they start to mix, thus making an amp less detailed. If i am wrong, please, by all means, correct me.

But if i am right, the amp with less distortion would be the amp with more details and better instrumental separation.

Could someone shed some light on this?

Yes ...🙂

All amplifiers have distortion , all amplifiers have a sonic signature , the best ones sound like music , the bad ones sound bland, there is no such thing as a blameless amp , regardless of how much times it's said and bland and grey is a sonic signature...


Just saying .....

 

[a.wayne]

Blameless suggests pure (no blame) audio - no distortion. I'm sure that's obvious. That is the engineers' type of amplifier that may measure only in the parts-per million of distortion. It does not mean dull sounding but it does mean that there are no intended distortion effects as are evident in many audiophile quality amplifiers.

Imaging is the percieved relative position of instruments, voices etc. on the "sound stage". This is partly defined by the studio left-right mix but some people like to enhance this by whatever methods are available. Higher harmonics help to locate sound sources and the actual range of harmonics provides some sense of relative depth or distance from the listener and even vertical information. The fact that this does not mimic a real experience though, can also lead to some poor results as well as average and good ones for those who can't get the needed distortion spectrum realistic enough.

This is not just tone controls or EQ. Distortion is the desired effect that can enhance the whole stereo audio experience. There are plenty of high-end brands that put existing and probably induced distortion to some good effect in their products. Goldmund would be just one.

Success with this will be pretty closely guarded by those who make this their business though, so don't expect more than generalised comments on how to achieve this. Few of us are much good at it anyway, from comments I read but there are commercial manufacturer forums here that will sell you good products.

From another perspective, these enhancements don't often scale up to high power systems very well and certainly don't suit multi-channel applications where there is interference with the specific processing used for Dolby/DTS or other synthesized codecs. However, if you are considering only a personal stereo system for domestic use, a good design like a small SE class A design or product, can lead to an extremely good experience. It's a pity that you can't just make more sophisticated designs for yourself but anyone who knows how to do this reliably is going to want serious money for their ideas.

If you really want a high power system, it would be better to go with reputable big amp designs as already discussed. These are not the ultimate in stereo performance but have the reputation of being good to listen to whilst having plenty of smooth power to compensate. The other very important consideration is that taking on DIY projects will soon change your wish list of what you want anyway. You don't yet know what you will get with any design, regardless of cost, power or advice, so kept reading threads before deciding you want this or that or you like this guy's opinions etc. Don't assume anything you read in this short time will remain interesting to you for long, it's a huge field to choose from. 😎

Specifically which high end amps are you talking about and more importantly where can i find one of those zero distortion amps you're talking about...



🙂

 

[sakellogg]

"zero distortion amps you're talking about..."


My first amp I made was like that! 0 distortion, no color, well to be truthfull I couldn't get it to work!

But it didn't add a thing to the music! Not even gain.

 

[still4given]

I asked Carlos about the changes he made to the Blameless design. I must say I liked his answer. He said the Blameless design was intended to be a starting place. A very neutral sounding circuit that he tweaked to get the sonic picture he was looking for. Anyway, as I said I liked his answer so I have ordered some boards from the recent DX Blame MXIII group buy and am looking forward to building one. I would also like to build Kevin's tweaked circuit to see the difference. I hope that someone makes a single sided foil design so I can etch a pair of boards and build it.

By the way, my Superamp is playing nice now. Very revealing amp. Not always a good thing depending on how good of a recording you're listening to.

 

[a.wayne]

Hi sakellogg

I'm not suggesting we all use the small transformers found in AV receivers and low-end 2ch amplifiers. Why stop at "2x"? Why not "10x" if more is always better? Of course most people would scoff say that is silly; it's also impractical to have a 5kVA inside a stereo 250W amplifier.

There's always a point of diminishing returns and I'm suggesting that point is reached long before "2x" with typical class A/B designs. I read on this forum a post from a guy who designed amplifiers and the transformer specification was 70% of the combined output power on their high-end units.

I'm currently building a DX Blame MkIII with 20 output devices. I'm interested in this thread because the amps being discussed are similar. The DX can output 200W into 8R with very low distortion or 880W into 2R with low distortion figures also. My speakers are 8R nominal impedance. I guess most of the time they are drawing a few watts at most, but I like to have the headroom for the dynamic passages in the music. I also like the idea of driving 4R and 2R loads but would not need to drive them beyond (say) 200W.

Most of the guys who are building their MkIII's seem to be using 1kVA to 2kVA of transformers. But for normal listening levels in a living room it seems to be extreme overkill.

My current plan is to use a single 50V-0-50V / 500VA transformer. The rail voltages will be slightly higher than needed and specified. I've checked with the designer and this is OK provided suitable component voltage rating are observed. My assumption is that this way peak power outputs can be sustained for longer, once the rails start to sag.

The smaller transformer means I can build an enclosure is lighter, and importantly, more compact. I end up with better seperation between the transformer and the electronics for reduced EMI. The money saved will be spent on the filter bank, which, if my understanding is correct, has a bigger influence on how the power supply deals with transient power surges.

I'm approaching this next project as an engineering challenge, where every design choice comes with tradeoffs. Or I might be crazy to think about strapping a 500VA transformer to a 20 output amplifier might be a good idea 🙂

Hello ,

You will need a transformer at least 4 times your 8 ohm rating if you are serious about it , of course you will not see this being done in the mid-fi amps you have mentioned and yes the audible difference is very noticeable , better to spend on a oversize transformer than an oversize bank of capacitors .

500 watts @8ohm (both channels ) would work best with 2k of transformers ...

 

[TonyTecson]

why not get both if you can afford it?
in my own amp rebuild, a big traffo goes with a big filter bank...
4 x 47kufd/100volts Nichicon caps...

this is the power traffo used in my 250watter per channel super leach...
it has a 2inch center leg stacked to 3 3/4 inches for a VA rating of 1750 volt-amperes..

 

[a.wayne]

can be seen here....Pioneer Spec-2 | Owners Manual, Service Manual, Schematics, Free Download | HiFi Engine
on page 30, looking at the schematic, it shows 91 volts(75v)...
now what could this mean?
i interpret it as 91 volts standby and 75 volts fully loaded..
now that's about 18% sag....
and till today the Pioneer Spec 2 still sells for over U$2k....
that tells a lot about amps....

the notation on the schematic implied FTC sine wave power testing...
in actual everyday music listening, sag will never go anywhere near that level...

For what's its worth , i had one, new in the 70's, they were nothing to brag about and a 4 ohm or lower load would send it to the morgue ...

2k huh ...🙂

 

[TonyTecson]

that is why there is a secondary voltage selector switch to lower lower the secondary rails for 4 ohms in case you forgot about it....please look at the schemes again....😀

to be honest, i never seen one nor heard one, but this amp was one of the
first ones i became aware of aside from the Dynaco 400 in the late 60's...
the Spec 2 comes out on eBay from time to time...
here's the latest on epay...Pioneer Spec 4 Amplifier Vintage Audio RARE Find Awesome Quality Sound | eBay
Pioneer Spec 1 Pre Amp and Spec 4 Power Amp No Reserve | eBay

 

[Ranchu32]

Hello ,

You will need a transformer at least 4 times your 8 ohm rating if you are serious about it , of course you will not see this being done in the mid-fi amps you have mentioned and yes the audible difference is very noticeable , better to spend on a oversize transformer than an oversize bank of capacitors .

500 watts @8ohm (both channels ) would work best with 2k of transformers ...

Why do I hear a request for a repetition of my advice?

For the last few years I always recommend Transformer VA of 1times to 2times the total maximum output power.

Simple to remember and simple to apply. There is some arithmetic involved, but any primary school pupil should be able to cope.

a.wayne, you say I need a transformer at least 4x the 8R rating while AndrewT suggests that 1-2x. Which one is it? Why is 4x the magic number?

Do I really need a 2kw transformer to listen to Billy Joel while sitting in an armchair, enjoying a glass of wine, sitting 4m from the floorstanders, which are rated 91dB/m/W in a 40sqm living room? If I turn up the volume more than a fraction my wife complains, or if the baby is sleeping, she complains.

 

[Ranchu32]

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AJT, may I ask where you purchased your transformer from? I can only find torodals in the bigger sizes in my part of the world.

 

[AndrewT]

Ranchu,
you are not comparing apples/transformers.

I said 1X to 2X total maximum output power.
A.W. said 4X.
4X 500W = 2kVA
1X (500+500) = 1kVA
2X (500+500) = 2kVA.

For continuous duty at 500W+500W the 2kVA will run pretty warm.
For music reproduction in a domestic environment the 1kVA will run cool.

You choose 1X or 1.5X or 2X. There is not that much between them.

 

[Johan Potgieter]

Yes ...🙂
All amplifiers have distortion , all amplifiers have a sonic signature , the best ones sound like music , the bad ones sound bland, there is no such thing as a blameless amp , regardless of how much times it's said and bland and grey is a sonic signature...

A.wayne,

With respect, let us revisit the definition of a blameless amp: 'An amplifier which adds nothing audible to the signal'. Nowhere did it say (Douglas Self) that the amp should be distortionless (meaning technically). In the business of audio, you will agree that there is a limit to what hearing can analyse? Surely it becomes academic what distortion/addition exists below that level (and obviously including that such products do not intermodulate to produce something audible)?

When one uses the term 'sonic signature', one per se refers to what is audible; i.e. sonic viz-a-viz human hearing. Then there are amplifiers that are blameless. The description of the 'sound' of an amplifier will at best be subjective. There is no meter calibrated in 'musical', 'bland', 'grey', 'warm' etc. I am also wary of such descriptions, e.g. 'musical' quite often points to the presence of a small quantity of low order harmonics, as has been shown many times.

One then enters the field of high niceness instead of high fidelity (as per dictionary definition). Some folks appear to have difficulty in deciding whether they want hi-fi or hi-ni. (I still remember the priceless piece of promotion, where the manufacturer said: "Wev'e added just enough distortion to make the music sound pure.")

 

[Ranchu32]

AndrewT, the case a.wayne cited was 500 watts for both channels (i.e. 250 watts per channel), which by his measure would require a 2kW tranny.

My power target is 200 watts per channel into 8R, 400 watts total, using the Dx MkIII design. Assuming I use one transformer for both channels, do I need:

400VA to 800va transformer, or;
800va to 1.6kVA transformer

I'd actually like to use one 500VA tranny

 

[TonyTecson]

a.wayne, you say I need a transformer at least 4x the 8R rating while AndrewT suggests that 1-2x. Which one is it? Why is 4x the magic number?

Do I really need a 2kw transformer to listen to Billy Joel while sitting in an armchair, enjoying a glass of wine, sitting 4m from the floorstanders, which are rated 91dB/m/W in a 40sqm living room? If I turn up the volume more than a fraction my wife complains, or if the baby is sleeping, she complains.

if you are doing sine wave power testing, 4x, or 2kw of power transformer is good to go...

if you are concerned only about playing music, then you can use 500watt traffos...

bottom line, your rails at 250 watts must be in the 70+ range..

 

[TonyTecson]

AJT, may I ask where you purchased your transformer from? I can only find torodals in the bigger sizes in my part of the world.

i made it myself....i design and build traffos for my own projects....😀
fyi that traffo is about 20 years old...