Amp Build Diary

[Chienmortbb]

I have taken inspiration from the 1 X 12 Cab Build Diary and as I am building my own amp, I thought other may like to watch the process and also read, and see, the reason I behind the choices I make.

My wife is very ill at the moment so progress in the build will be slow but hopefully the choices and explanations will be interesting. I am also looking for feedback so feel free to wade in.

At the end I will produce a precis of the thread so that no one has to wade through they whole thing. My next/first real post will be the specification.

Edited October 28, 2014 by Chienmortbb

[Pentode]

Sorry to hear your Wife is ill....... Best wishes to you both.

Look forward to your build.

[88reaper88]

I can only Echo what Pentode said.
Looking forward to your build and all my best to your wife

[madshadows]

Best wishes to your wife and good luck with the build

John

[Subthumper]

Hi, I'm looking forward to this as I've done a few of my own and am interested to see what someone else's approach is. Is it going to be valve or solid state? All my stuffs been valve but a lot of the process will be much the same. Only problems I had were staying up late working on the damn things,"just one more capacitor then bed" always turns into another hour. The other major thing I'd say is don't start until you have all the bits, as coming back to something either turns into a mistake or gets forgotten or holds up other things when you have to work around a shortage.
Oh and it's addictive. By the time your halfway through you will be planning the next one and working out how you will do it better than this one.
Good luck, it's incredibly rewarding.
Cheers Just

[Chienmortbb]

Thank you to for the encouragement. The detailed design spec is not complete but the following are planned:

1. All Solid State with a Class D power amplifier topology. The main reason is that Class D makes for easier thermal management,.To put it bluntly it is easier to keep cool.

2. Power output a minimum of either 250 Watts Average into 8 ohms and 500 Watts Average into 4 ohms or 2 X 250 (dual mono} into 8 ohms.Output on Speakons, no jacks for speakers.

3. Pre amp section. This was designed by Charlie Escher (passinwind on TalkBass) after several months discussion between him and me. Chalie has finished his amp but is constantly refining it.There is a short thread on TalkBass about Charlie's amp here
http://www.talkbass.com/threads/a-diy-500-watt-bass-head.1061473/

Like Charlie, I will be adapting the design but with his knowledge and consent. I should false emphasise that although I hade many discussions with Charlie, he did the circuit design, modelling of circuit behavior and built an tweaked the first iteration of the circuit boards.It is his design. More tomorrow

Edited October 29, 2014 by Chienmortbb

[LukeFRC]

[quote name='Chienmortbb' timestamp='1414625899' post='2591694']
Thank you to for the encouragement. The detailed design spec is not complete but the following are planned:

1. All Solid State with a Class D power amplifier topology. The main reason is that Class D makes for easier thermal management,.To put it bluntly it is easier to keep cool.

2. Power output a minimum of either 250 Watts Average into 8 ohms and 500 Watts Average into 4 ohms or 2 X 250 (dual mono} into 8 ohms.Output on Speakons, no jacks for speakers.

3. Pre amp section. This was designed by Charlie Escher (passinwind on TalkBass) after several months discussion between him and me. Chalie has finished his amp but is constantly refining it.There is a short thread on TalkBass about Charlie's amp here
[url="http://www.talkbass.com/threads/a-diy-500-watt-bass-head.1061473/"]http://www.talkbass....s-head.1061473/[/url]

Like Charlie, I will be adapting the design but with his knowledge and consent. I should false emphasise that although I hade many discussions with Charlie, he did the circuit design, modelling of circuit behavior and built an tweaked the first iteration of the circuit boards.It is his design. More tomorrow
[/quote] on the class D stuff.... do you know what module you're going to use? Question coming from.. they seem quite expensive to the general public....
My old Tecamp Puma 1000 used a Powersoft Digimod 1000 .... which is nice and self contained as far as I could see.... and the same module as in this £200 amp (apparently) http://www.altoproaudio.com/products/apx1000

[stevie]

Subscribed (as they say).

[Chienmortbb]

I expect some tough questions from Stevie after my posts on the 1 x 12 Cab diary but hopefully I can give the reason for my choices and who knows a good argument may lead to a change of direction but we will see.

The spec is taking longer that I expected ( or rather hoped so ) I will answer some of the questions that have already been put. Firstly the second build is already planned and it will be a no compromise rack mounted version of this project. The pre amp will have a separate power supply along the lines of passinwind's 500 watter on TB.

However back to this design. The power amp

As with the preamp, the power amp is not one circuit element but several. All basic power
amps can be broken down into the amplifier section and the power supply.

The amplifier section will be a Class D amplifier. There are several myths and misconceptions about Class D amplifiers. The first is that D stands for Digital. Amplifiers classes were named in order of their development. Class D amplifiers are switching amplifiers where the output mosfets (metal–oxide–semiconductor field-effect transistor) are driven by a PWM (Pulse Width Modulated) waveform that is analogous to the input audio signal. As a switching amplifier, Class D is very efficient with many amplifiers outputing 90%+ of the input power as audio.

Amplifier Classes go from A to H but not all are suitable for Audio. Classes that are suitable are A, B, AB, D, G and H.

All these classes can be implemented with Valves, Transistors or MOSFETs but fir Class D the ususl cloise is thr Poeer MOSFET. Sll commercial Class D amplifiers use MOSFETs,

Edited October 31, 2014 by Chienmortbb

[Chienmortbb]

Power supplies can be either limear of switch mode.

Linear power supplies use a transformer to down convert the 230V ac mains into a more usable voltage, they are simple but the downside is the weight.

SMPS or Switch Mode Power Supplies do not require a large heavy transformer but are more complex. Like Class D amps, grat care must br taken avoid RFI/EMI interference with audio equipment.

So the options are to have separate amplifiers and power suppliy or an integrated unit.

Edited October 31, 2014 by Chienmortbb

[stevie]

This is probably a daft question, but couldn't you modify a PC switched power supply? They are relatively cheap and well proven.

[6v6]

So what modules are you going to use then?

I looked into building a mosfet amp from scratch and the cost of the parts made it uneconomic, and as has been stated the class D stuff like Hypex is expensive too.

In the end I bought some used BK 450w modules from bertbass (a year ago! )[size=4], which I'm sorry to say haven't yet been built into the amp I had planned (hybrid with a valve preamp) due to work/kids time pressure. [/size]

[size=4]They are sitting under my workbench quietly mocking my lack of DIY commitment though [/size]

[size=4]Hopefully I'll get back to the project soon as building amps/cabs is a lot of fun.[/size]

[6v6]

[quote name='stevie' timestamp='1414761323' post='2593036']
This is probably a daft question, but couldn't you modify a PC switched power supply? They are relatively cheap and well proven.
[/quote]

I don't think the output voltages are high enough

[stevie]

True, but that's why I said 'modify' - maybe bump them up from 12V. Or maybe that's impossible. I'm just here to learn .

[Chienmortbb]

Most SMPS power supplies can be adjusted +/- 10% so the 12V supply could go to 13.2V but that is not much. A PC power supply ony has a lot of power available from the +12V rail. The -12V rail often has only 0.5A and the 5V line 1A or 2A.

With a 12V rail, the maximum you can get with reasonable distortion figures is 4.5 watts into 4 ohms ( in the old days all car stereos were 4-5 watts per channel). You could connect two amps in Bridge Mode to get 9 watts into 8 ohms and four amps in Parallel Bridge Mode to give 18 watts into 8 ohm. Your car stereo will probably say much higher figures and I will cover that later. Suffice to say that the above figures are calculated from the physics or the limitations of semiconductors.

So we need more power and that means more voltage. In my opinion you need at least 80V or (40V-0-40V) . This gives 200W into 4 Ohms, 400W into 8 ohms Bridged and 800W into 8 ohms Parallel Bridged.

Edited October 31, 2014 by Chienmortbb

[Chienmortbb]

Some of you may have come across Class T amplifiers. There is no Class T but some people use this to denote amplifiers using Tripath technology. Tripath were Class D innovators (sadly the company is now defunct). Tripath had their chips made in the far east by third parties and they are still manufacturing those designs. They are now old technology and are really Class D.

Edited October 31, 2014 by Chienmortbb

[6v6]

[quote name='stevie' timestamp='1414764675' post='2593089']
True, but that's why I said 'modify' - maybe bump them up from 12V. Or maybe that's impossible. I'm just here to learn .
[/quote]

Ok, apologies, my previous response came off rather curt as I was in a hurry and didn't explain, so let me see if I can remember and explain the theory

Ultimately, the output of the amplifier consists of an AC voltage (the signal applied to the speaker), which pushes current through the speaker (current determined by the impedance, so lets say nominal 8ohms)

If, for the sake of argument, we want 300w RMS, and we assume the speaker signal to be a sine wave, this means we can apply a simplistic estimate based on ohms law:

Power = Voltage x Current (P = V x I)

Voltage = Current x Resistance (V=I x R)

So if we combine the above, P=(VxV)/R, or RMS voltage = square root of power x resistance, so sqrt(300*8), which is just under 50v RMS

we multiply by sqrt(2) to get from RMS to peak-to-peak, so you're looking at, roughly, 70v p-p swing to push 300w RMS through 8ohms.

That's my understanding of the theory anyway, if I'm remembering correctly!

[6v6]

So, to clarify, it's pretty likely to be impossible to modify an ATX SMPS which outputs 5 and 12v to give the necessary voltage swing.

The other issue is SMPS technology is really complex, and well beyond what even pretty experienced electronics DIYers are likely to be able to modify (particularly if, as in this case, it'd effectively be redesigning the whole power supply..)

That complexity is one reason why I personally prefer simple traditional power supplies and either valve or mosfet amplifiers - it's just much more accessible to a hobbyist and if it breaks, I stand a reasonable chance of fixing it, which is unlikely to be the case with either SMPS or class D technology (not saying it can't be done, just that it's really hard for non experts without specialist equipment and knowledge)

[Chienmortbb]

[quote name='6v6' timestamp='1414834220' post='2593727']
So, to clarify, it's pretty likely to be impossible to modify an ATX SMPS which outputs 5 and 12v to give the necessary voltage swing.

The other issue is SMPS technology is really complex, and well beyond what even pretty experienced electronics DIYers are likely to be able to modify (particularly if, as in this case, it'd effectively be redesigning the whole power supply..)

That complexity is one reason why I personally prefer simple traditional power supplies and either valve or mosfet amplifiers - it's just much more accessible to a hobbyist and if it breaks, I stand a reasonable chance of fixing it, which is unlikely to be the case with either SMPS or class D technology (not saying it can't be done, just that it's really hard for non experts without specialist equipment and knowledge)
[/quote]One of the debates I have been having with myself is whether to go SMPS for the power supply or transformer. Of course the weight will be higher with a TX but the complexity will be lower. In general, fewer components means higher reliability.

As has already been mentioned, several companies make combined Class D with Switched Mode Power Supply modules. ICEPower, Powersoft, Anaview/Abletech Pascal and numerous Chinese manufacturers.They are great but also in some ways they are restricting.

As for Class D amps, they are now much easier to to understand and many hobbyists now design their own. However I am not going to do that.

[stevie]

Thanks for the clarification. This is already becoming quite informative.

[Chienmortbb]

The commercial amp builder will buy everything in bulk and will often fit lesser parts because the difference in performance will not be very much. The DIY builder will not compromise and that costs.

The one thing any DIY amp builder will tell you is not to start it because you want to save money. It will be more expensive than a shop bought amp. I was reminded of this today. Last week Charlie sent the PCBs for the preamps (5 boards and a few assorted parts). Each of the PCBs costs about £13 but they are really good quality. He also got some Bourns Cermet pots from Ebay that retail at about £10 each, for a really low price. He added those to the package and I got the card from Royal Mail to say they had arrived. Great but the downside is that I have been stung with £14.30 for Customs Charges and an £8.00 handling fee by Royal Mail. Be aware that if you buy from outside the EU ( i.e. USA or China) you will get stung. They add postage and anythng above about £15 has 20% VAT added to it. There is another limit (about £136 I think) where import duty is added. So watch those "bargains" from abroad.

Edited November 4, 2014 by Chienmortbb

[Myke]

Everything I've read so far has been WAY over my head but.. I am looking forward to the pictures

[VTypeV4]

[quote name='stevie' timestamp='1414683652' post='2592227']
Subscribed (as they say).
[/quote]

+1

I'm reasonably happy understanding valve gear and it's details but Switch-mode supplies, multi-vibrator circuits and Class D amplifier operation is somewhere just beyond me at this time.

Loooking forward to some pix too..

[Chienmortbb]

[quote name='Chienmortbb' timestamp='1415090767' post='2596464']
[url="https://www.dropbox.com/s/6hle9nv088nxx72/2014-11-02%2021.16.53.jpg"]https://www.dropbox....%2021.16.53.jpg[/url]

OK I have tried several way of uploading for ages and have failed. Help anyone?
[/quote]

[stevie]

Try this: http://tinypic.com/?t=postupload

Upload your image here and you'll get a URL to post in your message. Works great.