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Old 26th Aug 2020, 9:26 am   #41
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Default Re: Hope to start an audio amp with CV415/CV4046/TT15

Hello Andy;

You confused me to start with mentioning cathode resistor.
I am using a 47uF cap on the circuit, the radford diagram I am basing this lot on has a 50uF.

But suspect my like for a bit more base is down to the book-shelf speakers in use they can not be that good at the lower end. Simple Panasonic SB-14 boxes, probably supplied with a small system. But at something like £18 delivered from the Bay they were cheap.

I find it difficult to see lower powered speakers, all seem to want over 100 Watts drive which would be a waste to me, or at least I think so.

Adrian
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Old 26th Aug 2020, 1:08 pm   #42
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Default Re: Hope to start an audio amp with CV415/CV4046/TT15

Here is my attempt in Kicad at the drawing some of the resistor values will depend on the final PSU voltage, but I am awaiting a transformer person to answer a query.
I need to start asking understanding the NFB better.

But I should be able to at least start on a PCB layout.

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Old 27th Aug 2020, 2:31 pm   #43
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Default Re: Hope to start an audio amp with CV415/CV4046/TT15

Sometimes re-positioning speakers can deliver more bass, but as mentioned increasing C3 will also do that, as will increasing C4 and increasing C5. C5 as is, forms a LPF to make the stable at LF @ 33hz, increasing it to 47n will allow more low end through. R2/C2 & R8/8 & C7 all roll off HF and might need tweaking, it all depends on the OPT you use.

Having said all that, your frequency response is pretty good as is, so no changes to the amp are probably needed, but maybe better speakers. Technics did a brilliant series of mini hifi separates, their little speakers were brilliant, oodles of bass for their size, worth keeping an eye out for a pair.

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Old 27th Aug 2020, 3:04 pm   #44
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Default Re: Hope to start an audio amp with CV415/CV4046/TT15

Hi Andy;

Now I would have said that C5 was a high pass or HF boost from the 1st stage to the Pentode of the ECF82, it goes between the anode of V1 to the grid so to me more signal would get though the higher the frequency?

For instance I worked out that at 1KHz the 4n7 was Xc ~33K but at 20KHz the Xc was 1K6 so to an a.c. signal was expecting it to boost higher frequencies.

C8 I will admit to adding in there, Xc for that being around 300K at 20,000 Hz and 4K at 1.5 Mhz where I had an issue with self osc at 1.5MHz.

The transformers are from Primary Windings dot com, they have the taps at 20% although I am not using them on this obviously, but they should be good, there were designed for the Mullard 5-10.

Kicad is doing my head in so about to go for a drive to CPC.

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Old 28th Aug 2020, 8:18 am   #45
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Default Re: Hope to start an audio amp with CV415/CV4046/TT15

AFAIK C5 & R10 are a LF gain shelving network, but I could be wrong. I use them in my amps to stop any shennanigans at LF.

Sorry to hear about Kicad, i just installed on my Mint PC after being impressed with your schematic, but it sounds like I'll be sticking with a pencil and ruler having a low threshold for PC software idiocy.

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Old 28th Aug 2020, 9:39 am   #46
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Default Re: Hope to start an audio amp with CV415/CV4046/TT15

My problems with Kicad are the footprints for the components, that and I am not sure about the method to include things like transformers and speakers but to not include them on the PCB. So I started to just have connectors in the diagram and miss out the transformer and speaker or any controls thinking these would all be external to the PCB anyway. I use Kicad 5 and most of the documentation I had is 4.

I found a good series of videos on Youtube on this latest version this would get you started, it did me.
https://www.youtube.com/results?sear...Basement+Kicad

Will post more in a few minutes but need to get something completed.

Adrian

Just to add a bit more here attached I have added my latest diagram that I am working from, one of the difficulties in Kicad is valves are not that well catered for, you will find EL34's and ECC83 in there but few others, so for the TT15 EF81 ECF82 I needed to add these, then you also find you have to add the footprints for the valve holders and if you use the existing valve holder footprints, as I found out when trying to lay out the circuits they are done with the valves on the same side as the rest of the components as I had it on my 6021 milliwatt amp. But on this I want the valve holders on the back of the PCB to the rest of the components so that the valves can be above chassis board under the chassis. This is my sticking point as to haw I do it, do I reverse the pin directions or find another way. There is a video i have to re-study again.

It is frustrating but it keeps my brain active, the frustration is my inability to keep the information in my head, my learning process has slowed considerably from my youth.
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Old 28th Aug 2020, 11:33 am   #47
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Default Re: Hope to start an audio amp with CV415/CV4046/TT15

Long post and this was done just for my interest.

I hope this works, the spreadsheet I use is LibreOffice so saved as ods, plus a save as xlsx.

In my years as a service engineer, I never had to worry about designing circuits. I had diagrams to follow when repairing something, I could fault find, replace components to get signals back etc. Yes I could do simple circuits and had a range of standard values I would use when doing them. But the idea is to progress beyond that, put theories into practice and start building things with older tech, Valves.

This means dealing with high impedance circuits were capacitors values really do make a difference. I have no spice, or circuit simulation software and would not know how to use it.

So with Andy's comment about LPF, me thinking high pass etc, I was not sure what is what. So last night I tried to sit down and work things out on paper for just the first stage of the amp. I gave up with paper and tried a spreadsheet.

Assuming a.c sine inputs at various frequencies.

I have made assumptions listed here and this is where I would like advice as to these being correct?

1) Assuming a.c sine inputs at various frequencies.

2) From the anode of the U1 there is a potential divider to the grid of U2 this is made up of C5 & R14 in parallel and R15,R17,C10 and any grid capacitance in parallel, going to ground. The R17 C11 junction taken as a.c. ground.

So in the spreadsheet C5 parallel with R14 = value Rb
R15||R17 = 500K, 500K parallel with C10 = value Rc (add grid capacitance into C10)
The potential at U2 grid becomes a.c. volts at Anode * Rc/(Rb+Rc).

3) Dynamically the anode load Ra of the valve will be it's anode resistor, in parallel with Rd = (R2 and C2 in series) and in parallel with the potential divider (Rb+Rc).

4) So voltage gain will approximate gm * Ra and the ac voltage at the grid of U2 will will be dependant on input volts * u * Rc/(Rb+Rc).


Anyway I stuck it all in multiple columns and worked out in graph form the gain of the first stage and what I think would be the response of the gain and voltage on U2 grid. It allows me to visualise things with changes in values. What I can not even attempt to do is to continue with this adding other stages and then adding negative feedback to the system, I would not know how.

Anyway would appreciate guidance.

Adrian
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Old 28th Aug 2020, 2:53 pm   #48
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Default Re: Hope to start an audio amp with CV415/CV4046/TT15

Just touching on a point to do with NFB, Negative being the operative word here, so say you want to boost the bass ? cutting the treble is sort of equivalent yes ?
So having a high pass filter within the NFB loop allows more high frequency through said filter, ok so far so good, however as this is in the negative feedback loop this will in turn reduce the higher frequency gain overall, remembering this is Negative feedback here and so it works backwards.
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Old 28th Aug 2020, 3:41 pm   #49
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Default Re: Hope to start an audio amp with CV415/CV4046/TT15

Having a highpass function (a zero) in the feedback branch tends to add a bit of phase lead to the overall loop gain. As the loop gain and bandwidth get somewhat limited by the need to restrict phase lag around the loop, the added highpass effect is actually a good thing from the point of stability.

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Old 28th Aug 2020, 5:00 pm   #50
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Default Re: Hope to start an audio amp with CV415/CV4046/TT15

Quote:
I find it difficult to see lower powered speakers, all seem to want over 100 Watts drive which would be a waste to me
It's all marketing c**p, you don't have to drive them that hard, or even want to. My little Tannoy DC4s are rated for 50/200W I did measure the power supplied for comfortable listening, it was in the mW level.
 
Old 28th Aug 2020, 5:55 pm   #51
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Default Re: Hope to start an audio amp with CV415/CV4046/TT15

Quote:
Originally Posted by Red to black View Post
Just touching on a point to do with NFB, Negative being the operative word here, so say you want to boost the bass ? cutting the treble is sort of equivalent yes ?
So having a high pass filter within the NFB loop allows more high frequency through said filter, ok so far so good, however as this is in the negative feedback loop this will in turn reduce the higher frequency gain overall, remembering this is Negative feedback here and so it works backwards.
OK I can see that. I assume it is another potential divider from the speaker voiltage to the cathode of the first stage, get it correct and it will shut down the overall gain. In the 4 Ohm example it would tend towards a 330 Ohm to 100 Ohm leg as the frequency gets higher, so applying more negative feedback. I could also I guess change C13 and C15 from 0.1uF going a bit higher.

Quote:
Originally Posted by Radio Wrangler View Post
Having a highpass function (a zero) in the feedback branch tends to add a bit of phase lead to the overall loop gain. As the loop gain and bandwidth get somewhat limited by the need to restrict phase lag around the loop, the added highpass effect is actually a good thing from the point of stability.

David
Hi David, I have not even thought about phase shifts in the amp, I would have to get the scope going to view that and would need a few more scope probes for it.


Quote:
Originally Posted by merlinmaxwell View Post
Quote:
I find it difficult to see lower powered speakers, all seem to want over 100 Watts drive which would be a waste to me
It's all marketing c**p, you don't have to drive them that hard, or even want to. My little Tannoy DC4s are rated for 50/200W I did measure the power supplied for comfortable listening, it was in the mW level.
The ones I have now are 15 Watt shelf speakers which are fine in my small room and yes I can drive them with the 6021 amp which is around 250 mW and they are more than loud enough with me sat underneath them. But I was under the impression that speakers had a input sensitivity like anything else some quoting 85dB and some at 93dB for example, this being the output at 1 metre for 1 watt input. So for lower normal levels rather than be pushing several watts with the right speakers I may only need 1 watt etc. I am also guessing that some of the higher power units would have cross over units in them that would also loose a bit of sensitivity.

I had an accident with one of my speakers, it fell off the cabinet and landed on the 6021 amp. It flattened a valve to the board (all open chassis), it did not break it but I am positive I have a loose grid wire in there now as one valve suddenly became micro-phonic as well as damaging the speaker cabinet.

New valve on order.

Adrian
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Old 1st Sep 2020, 2:43 am   #52
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Default Re: Hope to start an audio amp with CV415/CV4046/TT15

Late night finish for me as I play with KiCad and the PCB design part.

I have used 1mm wide tracks for the things like power, i.e. HT and heaters. HT to the board should really be no more than 70 mA, but the heaters I am concerned with at 2.4 Amps.
Not sure if the tracks will deal with it, advice welcome.

0.5mm wide tacks for everything else.

Also using ground plane copper fill is that a good thing, again not sure if it adds lots of capacitance or not?

Component side
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Valve side so that valves can be on chassis.
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Board size is 182 x 114 mm at present.

Time for sleep and a think about it later.

Adrian
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Old 1st Sep 2020, 6:26 am   #53
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Default Re: Hope to start an audio amp with CV415/CV4046/TT15

Well done re the PCB. Re heater traces looking at my wire ampacity chart 24 AWG gauge will do 3.5A in free air, 24 AWG = 20 thou, that's as far as I got, i was going to work copper thickness of PCB etc but too many unknowns.

When I'm building an amp I know from experience what wire gauge to use, it looks right, I then measure V when built, if too low I up the wire gauge. which is a long winded way of saying suck it and see, if the voltage drop is too much beef the traces up with solder or solder on a piece of copper wire,, but it looks like you have solder mask on those traces, and they look too thin for the OP valve.

If your ground plane is one trace your ok, you'd just need to beef up one trace to the TT OP valve perhaps. you could cut out a track of the same thickness as your PCB on a scrap of copper clad board, apply a voltage to a resistor or better still use the valve, might be quicker than calculation and save tears later. To be honest not sure why you'd want to use a PCB, it complicates a simple job and has inherent issues due to heat. There's lots of threads out there where folk came a cropper using PCB's and valves.Every valve scope I've looked at has fried PCB's, and that's only with small signal valves not whacking great OP valves. Still, it's your amp, your build and you sound like you know what your doing.

RE LPF vs HPF/shelving network see - http://www.turneraudio.com.au/100w-monobloc2-2004.html - 2nd para under Figure 2. When i built my big 120w monoblocks I nicked a lot of ideas off this amp and corresponded with Patrick Turner, he mentioned C6/R11 as a way to improve LF stabilty, an issue I had with that amp. Not being 100% on my theory I try things then test, rather than work out on paper. I tested it, it worked.

Despite several attempts by David and others to explain NFB, poles, Nyquist diagrams and bode plots I still struggle with the subject, however everytime I build and test an amp, a bit of the puzzle drops in place, also I try to come at the problem from another angle. Pat Turner mentions testing an amp with a sweep and looking for where THD exceeds 2%, which is another way of finding where an amps response has too much phase shift or resonance I suppose. Also see this article - http://www.angelfire.com/electronic/...pensation.html

Hope that helps answer some of your question, Andy.
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Old 1st Sep 2020, 10:57 am   #54
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Default Re: Hope to start an audio amp with CV415/CV4046/TT15

Hello Andy;

Not sure on a few things in your comments,and this is most likely down to my understanding.

You say "If your ground plane is one trace your ok", can you elaborate on this please, the ground plane is on both sides of the pcb, top and bottom and there are interconnected with via's, little through plated holes.

I wonder if we are just getting mixed on terms with the low pass thing. The article calls it a "LF gain shelving network", you mentioned low pass filter, LPF, to me that is a high pass filter that would increase gain at HF therefore have less gain at LF, it could all just be different phrases for the same thing.

I will send an email top the pcb place asking about current of 1mm tracks, but I just found some on line information think that normal pcb track is 0.035mm thick and I can increase that to 0.075mm which is 1.37 thou or 2.95 thou.

So 1 mm trace will give me an area of 0.035 sqmm, or if I thicken the tracks 0.075 sqmm. Looking at the chart I have this is no where near enough. So I will probably have to change the pcb to have holes next to the heater pins to allow me to hard wire between them, that may be the safest way to do it.

As to why I use PCB's, well my fat fingers have issues at times and when I started to learn Valve tech I also started to learn KiCad so they are going together when I can.

I use JCLPCB and the price for 5 boards is as follows:-

Charge Details
Engineering fee:£2.98
Surface Finish:£0.82
Board:£4.77
Build Time: PCB: 1-2 days
Calculated Price: £8.57
Weight:340g
Shipping Estimate: £5.11 Via GB Special Air Mail(RoyalMail)
Delivery Time:12-16 business day

So it just seems an easy way to go of 5 boards for £16.42 (inc VAT) all in.

Adrian
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Old 1st Sep 2020, 11:05 am   #55
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Default Re: Hope to start an audio amp with CV415/CV4046/TT15

If things are laid out well, you can solder shaped pieces of tinned copper wire along tracks to boost their current capability. You just have to design in a slot in the solder mask to allow the soldering.

Be aware that individual stitches are likely to have quite limited current rating. It's normal to have an array of them where I high current connection has to pass through the board.

David
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Old 1st Sep 2020, 2:20 pm   #56
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Default Re: Hope to start an audio amp with CV415/CV4046/TT15

Hello David, I may struggle with that idea, I tend to struggle a bit with KiCad as I am just really learning with it. This will be my 3rd board and I try to push a bit every time.

I have just posted a question on their forum regards having a track top and bottom of the board to allow me to double up on the thickness, it does not seem to allow this, they will only be short tracks between the valve pins and pad holes. but will wait and see. If I can do it then I can use twisted pairs to hard wire the heaters. That will be safest for me, I think?

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Old 1st Sep 2020, 4:14 pm   #57
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Default Re: Hope to start an audio amp with CV415/CV4046/TT15

My question was answered so I now have a version 2 layout.

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So I shall now sit on these for a while to see if I can do better or figure out something else.

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Old 2nd Sep 2020, 7:18 am   #58
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Default Re: Hope to start an audio amp with CV415/CV4046/TT15

That looks a lot better. On your old layout it looked like one heater wire was your ground plane or was connected straight to it, if the GP is large it would carry more current than a 1mm trace. Heaters are supposed to be grounded somewhere and not float, hence my assumption.

I'll admit to getting confused re LPF/HPF terminology, the LF gain shelving filter is of coarse a first order HPF. My brain sees the word "high" in HPF and jumps to the conclusion that it cuts "highs". I've told it numerous times this is not the case, but it insists this is the case. I guess it's how our (some or our) brains work, mostly on automatic, filling in gaps and making large assumptions. Anyhoo, it's going to get a clip round the ear when i get home. Sorry for the confusion.

BTW five boards for less than a score is very good value and I understand why you would use one. It does make a project look more proffesional and neat.

Andy.
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Old 2nd Sep 2020, 7:56 am   #59
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Default Re: Hope to start an audio amp with CV415/CV4046/TT15

I would make all valve base tracks at least the size of the pad, for a short distance anyway. This reduces the thermal track breaking that can occur. And make all tracks a bit thicker, again the size of the pads. Some may not agree but for a simple board it gives a lot better mechanical result.
 
Old 2nd Sep 2020, 11:42 am   #60
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Default Re: Hope to start an audio amp with CV415/CV4046/TT15

I have noted the comments here from merlinmaxwell, (sorry not sure on your name).

I have generally thickened the tracks, minimum width is now 1mm rather than 0.5mm. Where there are tracks from valve pins I have made them 2mm as that is typically the diameter of the pad they will connect to, the only exception to that is a track that goes between valve pads reduced to 1.5mm.

The main HT tracks going to 2.5mm.

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The other side of the board has not changed.

The heaters I am using a 6.3V centre tapped output from the transformer, to help keep 'hum' down, the CT has it's own hole on the board.

There is one major advantage to getting them made and using KiCad for me, compared to the old days of etch resist pens, or track resist layout is that all the holes are plated through holes with solder pads on both sides of the PCB so mechanical strength is a lot better then I could ever do with home made boards.

I suspect the price will jump when I get 2oz copper, or double thickness tracks, but I will post the final costs for getting 5 boards made when I place the order.

A new scope probe has arrived from Farnell, so I will try and have a look at phase shift through the amp for a number of different frequencies, that may help me understand a few things, guess I am looking to keep a 180 degree shift best endeavours from input to output through the band, as my test set only goes to 20KHz that will be all I can up to.

It's all fun at times!

Adrian
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