Power amp under development

The pinouts shown on the layout are correct for a TO92 (cbe) mounted underneath the PCB, so looking through the top they are reversed.

For clarity this is how I mounted T8.

Cheers
 

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Re: T8 Pinouts

Hi Quasi,

I see you are quite the artist. Having seen your second example of artwork it is clear you are very good at doing technical drawings.

I was not aware you actually placed T8 into a bored hole in the heatsink. A very interesting and thinking outside the box approach for a TO-92 case.

The reason I had asked a while back of using an alternate to BC546 was I was not sure how I would be able to properly secure a TO-92 case thermally. That was why I asked about using a flat case type with a hole such as TO-126 would allow and, at least for me, easier to to ensure a sound thermal mount.

I know many in other threads, discussion venues and projects have bantered about the importance, or not, on the topic of directly mounting the T8 like tracking transitor on top of one of the output drivers or on the heatsink beside/between the output drivers. I am inclined to mount T8 ontop of one of the output drivers. I thought this would be the "safe" approach, hence my desire for a flat form factor with mounting hole for T8, such as a TO-126 like form factor.


Regards,

John L. Males
28 December 2005 02:05
Willowdale, Ontario
Canada
 
duran_1960 said:
I want to present my thanaks to whole friends for their all contributions to this sectionns. They were very usefully and frienllly. Thank you very very much again and celebrate your
holly days (Christmass) God bless you! ...


Hi duran_1960,

Thank you for your wishes. All the very best to you, your family and friends for the new year.

I'm glad you have found this thread useful. Are you building anything at the moment?

Cheers
 
Questions on Some Part Ratings/Values

Hi All,

I been searching parts stores locally here for capacitors and non-output device tranistors mixed in with some research on component functions. I know the parts for this used were largely used from what is available. ;) I can find most of those parts, but some other parts for basically the same cost are available that may merit consideration. I do not have the stockpile of existing parts other have. So I have the following questions to help me understand ratings or key ratings so I can decide if an alternate may be suitable:

1) What would be the reasonable minimum ratings for Vceo(V), Ic(A), PD(W), and fT(MHz) required relative to PSU voltage (like +Vrail + x, or % of +Vrail) for:

a) T8?
Alternates under consideration are 2SC4495, 2SC3964, 2SD669, BD139, MJE340.

b) T1/T5?
Alternates under consideration are 2SC2240, 2SC2547/6.

c) T2/T3?
Alternate under consideration is BC560.

d) T4?
Alternate under consideration is BC550.

e) T7/T9?
Alternates under consideration are 2SD669, 2SC4883.

f) T6/T10?
Alternates under consideration are 2SB649, 2SA1859.


2) Is C7 an Electrolytic or non-polar? What is the rated working voltage required related to PSU voltage (like +Vrail + x, or % of +Vrail)? Would a Rubycon 120uF electrolytic pose issue in terms of the 120uF value or fact it is a Rubycon? I have read lots of pros/cons to Rubycons and when to or not to use with or without circuit specified bypass cap. So far it is a challenge to find a 63V rated part. The Rubycon is tight fit for PCB that may need to be modified by me to fit. A Nichicon 63uF, 250V cap is available. I am not sure if the 63uF value will pose issues being lower uF or what that may imply using a lower value. For sure I will need to modify the board to enable the Nichicon stubby form factor capacitor to fit. I can still search for the 100uF 63V version, but I thought I mention the options just in case. I am aware of the likley 20%-25% tolerance electrolytic caps have, but I am ignoring that for the purpose of the question.

3) For C2/C3 can I use 470pF? I can buy some 470 pF ERO KP's for this. Or is it better to be a MKT type (whilst trying to skip the passionate discussion of MKT vs MKP)?

4) For C4/C8 is Silver Mica good, great, so so or do not use?

5) I have seen C11/C13 listed as 220uF, then 330uF for the 350W version, listed as 470uF for the 500W version. How does one determine what is best value for these on board supply PSU filtering caps is? I am assuming for the 500W version there was good reason to list as 470uF? I have good access to 330uF, 470uF, 590uF, 680uF and 1000uF. Reading I have done so far is suggestive of values towards 590uF, 680uF or 1000uF independant of power rating of amplifier. The 590uF is a Sprague terminal screw type. The 330uf will fit the existing board layout. I will and can modify the board to handle the larger diameter values. Thoughts here would be helpful.

6) Is C12 supposed to be 0.1uf or 0.068uF? I have seen a board layout a while back indicating a value of 0.1uF, but the schematic shows 0.068uF. The current board layout does not list the value for C12. Is not the issue for me if the value is on the layout, just which value should be use and/or what reason to use one or other value.

7) Regarding C1, I can use a MKT ERO 2.2uF 10%, or a MaxCap 1.0uF Polypropylene 5%, ERO MKP 0.68uF 10%, Rifa 0.68uF 5% or a yellow tubular 1.48 MKP 2% with logo that has a small triangle just above fitting notch of a retangle. Of course the design specifies 1uF. I cannot find any info on the MaxCap, so I am not sure if it is worth using nor if 24K plated leads implies good quality. Anyone know anything about MaxCap's? I am torn between using the MKT ERO 2.2uF, 1.48MKP and MaxCap. I am weakly inclined to use the ERO MKT for a known level of behaviour despite it being a 2.2uF. It seems 1.0uF and 2.2uF are common values used for the input coupling capacitor. Thoughts and any idea what the 1.48MKP maker is as it is a 2% tolerance using G on rating markings?


Regards,

John L. Males
Willowdale, Ontario
Canada
16 January 2006 15:21
 
Hi John,

I have attached the schematic again, this time with key current flows shown. This will allow you to calculate voltages for key components and with the current flow information, help to select components.

Example 1: the maximum voltage across T8 will be 12.5mA x 717 ohms (sum of R13, VR2 & R14) = 8.9 volts.

Example 2: The approx voltage across T1 / T5 = positive rail voltage - v(R4) - v(T2). with 70 volt rails this works out to be; 70 - 0.6 - 1.2 = 68.2v.

Example 3: The approx voltage across T4 = negative rail voltage - v(R7) - v(R6). With 70 volt rails this works out to be 70 - 0.6 - 46.8 = 22.6v. By changing R6 the voltage across T4 can be varied.

Have a go if you like at working out the other voltages.


2. I used a polarised electrolytic (I can hear the screams from other members as I type). You can use a non-polarised one if you like but because the DC voltage here is 0v (or should be) a polarised electrolytic can be used. 120uF or near value is fine, though the bass roll-off will be affected slightly. I don't subscribe to sonic differences between capacitor brands (is that more screams I hear?) just make sure it's a well known quality brand (rubycon, philips, nichicon etc.)

3. Yes ok.

4. I used a ceramic (more screams). Silver mica is ok.

5. Use anything up to say 1,000 uF, it doesn't really matter. The main job of this capacitor is to shunt broad spectrum ripple developed by heavy currents flowing through local tracks and the fuse. More is better here with diminishing returns though I doubt you will hear the difference between the 330uF and the 590uF. Do not however omit the 0.1 uF polyester / MKT bypass capacitors.

6. This is a basic filter to stop RF entering the amp from long speaker leads. It also provides a high frequency load to quench any oscillations (that shouldn't be there in the first place). It doesn't really matter so select what you have on hand, just make sure its rated to at least the rail to rail voltage.

7. You can use anything you want here of the polyester / MKT variety over 1uF up to about 4.7uF if you can get it to fit. Only use a non-polarised capacitor here to provide protection against faulty connected pre-amp. Tolerance matters diddly here.

Of course comments from other members is welcome, I suspect there will be a few.

Cheers
 

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  • nchan mos 350 schematic 1v4.pdf
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edl said:
Anyway Dear Quasi,
what about

- adding a 100nF foil cap paralel with T8?

Regards,


Edl,

I will jump in here.

T8 does have a 0.1uF (100nF) cap across CE, on the PCB. In error it is not on the schematic last issued. I have an updated schematic with this cap that is on the PCB. I called it C14 so the other cap part labels do not change. I have not posted the update pending some other possible notes/changes I have not had time to ask about or do. I been busy and so stage "advance" updates.

References:

Post #253

http://www.diyaudio.com/forums/showthread.php?postid=787264#post787264

Post #267

http://www.diyaudio.com/forums/showthread.php?postid=788099#post788099


Regards,

John L. Males
Willowdale, Ontario
Canada
17 January 2006 14:14
 
edl said:
Hi AndrewT,

0,6V/47ohm=12,77mA, so 12,5mA semms OK to me.

Anyway Dear Quasi,
what about
- adding a 470ohm resistor in series with T7's base;
- adding an 1-10ohm/2W resistor paralel with L1;
- adding a 100nF foil cap paralel with T8?

Regards,

Hi edl,

What is your reasoning for the 470 base resistor?

At 20khz the impedance of the coil is about 0.5 ohms. Adding the 10 ohms in parallel will have very little effect.

Thanks John for answering the last question. One day I'll fix the schematic.


Cheers.
 
AndrewT said:
Hi Quasi,
can you check the current through R12 & R15? 12.5mA seems too high.

Nice idea showing currents on the schematic.
How about adding input bias current/voltage and gate voltages?

Hi AndrewT

At idle the approx. gate voltage is 18.1mA x 220ohms = 4 volts.
At idle the voltage across the output source resistors = 0.14v for 0.47 ohm resistors or 0.1v for 0.33 ohm resistors.

Is this what you meant?

Cheers
 
Power Supply PCB

Owyagoinfellas,

I was asked by email for the PCB layout that I used in my amp.

I originally did not post it because power supply layouts can vary a lot depending on many factors i.e; case used, parts available and personal requirements such as quality, looks etc.

But I was asked so here it is. The PDF shows the layout and tracks on two pages. Remember to reverse the tracks.


Oooroo
 

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Hi Quasi & Edl,
thanks for the confirmation, my mental arithmetic must have gone walkabout.

The voltages and currents around the Vbe multiplier seem a bit unusual.

For a gate voltage of 4V the Tr8 Vce is about 7V to 7.1V. This requires a resistor current (R13 R14 VR2) of about 10mA leaving only 2.5mA through Tr8. Does this low collector current suit a particular characteristic you need from the multiplier?

The 30mA through the Rs produces a quiescent voltage of 10mV. or is it 300mA?

I meant at the input to the LTP. Input bias current and input bias voltage developed across R3.
 
AndrewT....

Duh...what's a decimal point between friends...hmmm?

Yes it's 14mV and 10mV :eek:

Regarding the current through T8, this was just a guesstimate while I played around with R13 and R14 during the amps development. I had no ideas about the characteristic of the thermal tracking of T8. I just fiddled until I liked what I saw. As it turned out the thermal tracking responds conservatively but adequately to maintain the set bias current.

I guess the input current through R3 to be around 18 uA. If anyone has the lid off their amp at the moment can you please measure the voltage across R3 (33K).

Cheers
 
Power Supply and Vceo Calculations

Hi Quasi,

Thanks for posting the PSU design you used. At least from the PCB one now knows what PSU design you used. I have two simple questions. Am I correct you used one PSU board per channel in your amp (implied from the pics you posted of your amp)? The second question is more a curiousity, is there a reason you choose to place the bypass caps and PSU output near the rectifier as opposed to many designs placing at edge oppsite end to the bridge/transformer input? Basically this second question about the PSU is if the primary reason was related to some sort of quality of the PSU output or just a preference/habit you have in PSU layout.

Thanks for the info and example on the Vceo questions I had, as well as the other answers of my posting. I am still "digesting" the Vceo part of the answer. Many are straight forward to calculate based on examples and know knowing what the current flows your marked. The "digestion" surrounds two areas. I am trying to see if I can figure out using the math and summing up the voltages to sort out one area that is not clear. The second may be answered by first activity, or I will need to ask a question about that as well.


Anyone,

If anyone has the cover still open on their version of the amp I like to know what the voltages while amp is idle (no input signal) with speakers connected to help me figure out some of the calculations I am doing:

1) Between Collectors of T6 and T7?
2) Between Collector and Emitter of:
a) T8
b) T4

And if you are willing:

c) T1
d) T5
e) T2
f) T3
g) T6
h) T7

3) Across C7 andnoting if it is an electrolytic or non-polar you used?
4) + and - DC rails

I will need to know what value of R6 you choose to install (no need to measure with a meter).

If you used different transistor(s) that listed on the schematic for anything but the output devices please note part number from schematic and the decice chosen. I will assume the "C" version of the BC parts was used unless stated otherwise.

I would be most grateful for any responses for these voltage measurements of working amps.


Regards,

John L. Males
Willowdale, Ontario
Canada
19 January 2005 01:42
19 January 2005 01:45 (Typo mistakes corrected)
 
quiescent current problem

hi quasi !
thanks for your job and your answers.

i've realized my own pcb with your schemetic becaus my heatsinj and the free place are special.

it's working but i got some probllem with quiescent current and voltage bias between t8 CE.

the first time i 've used some igbt for output devices,it s working

but the thermal stability is awful, quiescent current go up and down dramatically so i've changed igbt for mosfet (fqa 44n30 fairchild )

the fist time i started the amp with +-45 v temporary supply
i ve changed r13 to 1200 ohms to get a correct voltage for Vgs (about 5 volts to run 100 ma per mosfet )

but after a few moment,i ve checked the small heatsink and i ve found it very hot with this low votage supply (+-60 v will be worst )
so i decided to change current across r15 (now it s 100 oms )
and i ve changed r20,r22 to 510 ohm to reduce power dissipation in t6,t7,t9,t10.
ok temp seems to be correct (35 ° with 20° ambient )

but now i cannot get mote than 3 volts across r20 and r22

do you know if the new current in t7 is too low or i have to change some resistors values ?
 
Hi thierry,

Does your amp work. I mean have you heard clear sound through a speaker.

If it does work then may I advise the following.

Changing R13 to 1200 ohms will not help the heat situation much and will alter the characteristics of this section. I suggest you put the 470 ohm resistor back in. The small heatsink normally runs very warm, almost too hot to touch, but the MJE340/350's are designed to take it. Take the time to look at the datasheet and you will see that these transistors can still dissipate a lot of heat at high temepratures.

But if you are worried about the heat you can increase R12 & R15 to 68 ohms, these must be the same. This will decrease the second stage current by 3.5 mA and will reduce the heat produced by T6 & T7 by 0.4 watts.

The only way to reduce the heat on T9 & T10 is to increase R20 & R22. I suggest using 330 ohm resistors to reduce the current through these by 6mA. The power dissipated by these transistors will then drop by about 0.6 watt.

So in total that's a saving of around 1 watt and it may be enough to make you more comfortable.

Once this is done reset the bias using VR2. I only use 30mA per FET pair, in my opinion there is not enough benefit in running 100mA, but it's a personal choice. Remember though that 3 FET pairs with 100mA bias current each on 60 volt rails equals 36 watts continous dissipation, so even large heatsinks will get quite warm.

Anyway, thanks for choosing this design. If you have the time please send me a photo of your work.

Cheers & Good Luck

Quasi
 
Hi John,

Yes I used 1 PSU PCB for each amp module.

My rational in capacitor placement is to avoid voltage drops over lengths of track. So I place my by-pass caps and output connectors in a position where ripple currents flow away from them. So yes there is a reason.

My amp is out on loan again, taken tonight in fact.* So I cannot provide measured voltages only mathematical ones.

Cheers

* Amp enjoyment hours by others exceeds Amp enjoyment hours by quasi by a magnitude of about 10. :(
 
power dissipation

hi,
sorry,the first step was
-r12 r15=100 ohms
-r20 r22=510 ohms

heatsink is ok after this change

but i need to put 1200 ohms instead ok 470 to get a normal Vgs (about 5 volts )

i don't know if it's normal,have you simulate your amp ?
if yes,can try to change values as i did.

i will post square waves at different frequency to show the response