[Kal525]

alright, i've spent about a week and a half trying to come up with a reasonable parts list for this amplifier. PLEASE critique the hell out of this thing. I wanna know what's wrong with it before i actually order the parts.

It will be used for a home setup, wall powered, with sennheiser-hd600 headphones and a sound card as the source.

- AD8620 op-amp. I think i need 2 of these, tangent, on your site, it says you sell this mounted on an adaptor and tested. Is it just one of them mounted, or do i need to purchase 2.

- C1 1.0 uF recommended by JMT, but can i find one that fits?

- C2/C3 4x Elna Cerafine 470µF/25v

- C4 2x Wima MKS-2 6.8µF/50V

- R1 2x Vishay/Dale 1K 1% 1/8W resistor

- R2 2x 500K to 1M resistor depending on value of pot

- ALPS blue velvet pot, there supposedly exists a 50K version of this, but all i can find is the 100K at angela.

- R3 - R6 gain set to +13
-R3 – 332 ohm
-R4 – 4.75K ohm
-R5 – 2.49K ohm
-R6 – 221K ohm


- R7 50 ohm resistor

- R10, i've gotten two recommendations for this one 100 ohm, and 1K, those are pretty different and i'm not sure which is the right value.

- Cascode set from tangent for biasing op-amp into class A

- 3x EL2002 buffers

- 2 RCA phono input jacks with insulating nylon washers

- 1 neutrik locking output jack

- Blue LED with appropriate RLED value (probably 2.8 mA of current)

- Texus instruments rail splitter

- D1 or D2, from what i gather, you use one of these for reverse power hookup protection, but i think i've seen both of these used on picture on this forum for some reason. Anyone know why?

- Pactec LH57-200 case

I think that's just about everything, with the things i am unsure of noted as such. Also i liked the volume knob that tangent used on the meta42 he constructed in the hammond case, anyone know the model number of this?

Probably the thing i am still most unsure of is what i should buy to power this thing. Since i'm using 25V caps i think that means i can use a power supply of up to 50V which i obviously will not do. I was thinking that 18V would be good, but even if this is the appropriate voltage for my setup, i do not know how to power this thing from the wall. I'm not really worried about batteries, so i don't have to throw them in there in conjunction with the wall power if it is more difficult.

 

[Squalish]

Quote:

Originally posted by Kal525 - AD8620 op-amp. I think i need 2 of these, tangent, on your site, it says you sell this mounted on an adaptor and tested. Is it just one of them mounted, or do i need to purchase 2.

I have no real experience in this other than what I've read, but I believe that the AD8610 is 1-channel and the AD8620 is stereo. So you would need 2 8610's, or only 1 AD8620. The adapter is to mount 2 8610s on a board designed for 1 opamp, or vice versa, not sure.

 

[tangent]

Quote:

AD8620 op-amp. I think i need 2 of these, tangent, on your site, it says you sell this mounted on an adaptor and tested. Is it just one of them mounted, or do i need to purchase 2.

Squalish is right. You need two op-amp channels for stereo. Some chips come with two channels on one chip, others come singly only. The AD8620 is a dual op-amp.

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C1 1.0 uF recommended by JMT, but can i find one that fits?

If you want an easy answer, get the Panasonic ECQP 0.22uF one I recommend in the META42 parts table. If insist on doing something different, you'd better do some reading. The META42 article links you to some web pages that are relevant to this topic, in the C1 discussion section. Also, do a search here at Head-Fi in the DIY forum for "polypropylene polyester ceramic". Read everything you can find, and then decide if you think I know what the hell I'm talking about or not.

I'll tell you another capacitor that fits in C1: a Panasonic ECQE 3.3uF capacitor. Do you think that's better or worse than the 0.22uF ECQP? If you don't know the answer, you'd better start reading, or just take my advice and go with the ECQP cap.

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R1 2x Vishay/Dale 1K 1% 1/8W resistor

Vishay/Dale makes lots of types of resistors. The ones you want are variously called the CMF or RN55 series.

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ALPS blue velvet pot, there supposedly exists a 50K version of this, but all i can find is the 100K at angela.

You have to import the other versions. It's not worth it for just one part. The 100K version is just fine.

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R3 - R6 gain set to +13

Why so high?

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R10, i've gotten two recommendations for this one 100 ohm, and 1K, those are pretty different and i'm not sure which is the right value.

This resistor is there to isolate the input capacitance of the Q2 JFET from the signal passing from the op-amp's output to the buffer's input; the value is not terribly critical. The higher the better, up to about 1K. You could try a bunch of values, but if you want to be lazy, just get two extras of some other value in this range -- double your R1s, or your R3s, or.... I most often use 100 ohms, which I also use for R7.

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2 RCA phono input jacks with insulating nylon washers

I don't know this Pactec case you're talking about, but as far as I know, PacTec only does plastic cases. If it is indeed a plastic case, you don't need to worry about insulating anything from the case. I do recommend that you go with a plastic case for your first project -- they're much simpler to deal with.

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D1 or D2, from what i gather, you use one of these for reverse power hookup protection, but i think i've seen both of these used on picture on this forum for some reason. Anyone know why?

It would be silly to use both at the same time. Use one or the other, depending on the situation. The META42 docs describe the tradeoffs.

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i liked the volume knob that tangent used on the meta42 he constructed in the hammond case, anyone know the model number of this?

It's the Kilo ML series.

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Since i'm using 25V caps i think that means i can use a power supply of up to 50V which i obviously will not do. I was thinking that 18V would be good, but even if this is the appropriate voltage for my setup...

If you're using 25V caps, you might as well go all out and use a 24V supply. A good regulated 24V supply like the Elpac WM080 will maintain 24V under all rated load conditions, unlike a cheap power supply, and it will sound better than a cheap power supply besides.

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i do not know how to power this thing from the wall

The wall wart has two wires coming out of it -- these go to V+ and V-. If you use the WM080, the DC power jacks mentioned in the META42 docs mount on the case and break these two conductors out for you, and you wire them to the board just like you would the RCA jacks.

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I'm not really worried about batteries, so i don't have to throw them in there in conjunction with the wall power if it is more difficult.

Combining wall and battery power at the same time is more complex. Don't do it for your first project.

 

[JMT]

I use a 1.0µF cap in C1 based on Tangent's original parts list. I have used the 0.47µF as well, along with the 0.22µF. But, that was with v1.0 of the META board.

But, like I mentioned in my PM to you.....Tangent designed the board, he knows it better than anyone, go with his recommendations. Quote:

Combining wall and battery power at the same time is more complex. Don't do it for your first project.

Especially this.

 

[tangent]

Quote:

I use a 1.0µF cap in C1 based on Tangent's original parts list.

The beauty of a 1.0uF cap for C1 is that it results in a stupid-low input corner frequency even if you use a 100K R2 because your pot is 10K. However, if you use a 50K or 100K pot and your R2 is therefore also 5-10x higher, C1 can be quite a bit lower and still have great performance. That's why we chose to shrink C1 in v2 -- because we can get away with it. If you're worried that C1 isn't low enough at 0.22uF, you're probably the kind of guy who should think about jumpering C1. This ultimately comes down to a personality difference, so I can't provide you with absolute truth here.

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Tangent designed the board

It is inaccurate to say that I designed the META42 board, just like that. I helped, but what's more important is that it's my job to communicate its subtleties to people like you, Kal525. Nothing is set in stone with this board. 100 or 1K for R7? It depends. 100K or 1M for R2? It depends. D1, or D2, or both, or neither? Sometimes, sometimes, never, and probably not, respectively.

You seem to be looking for absolute answers, Kal525. Sorry, there is no absolute truth in audio design. There are obviously incorrect things, of course, but once you've discarded those things, what remains is still a vast field of choices.

 

[Kal525]

thanks for the input, and trust me, it's not that i don't like the choices, it's just that i want to make sure i'm getting what will sound best and not something that will totally screw it up.

I think i now understand how C1 operates. It's part of a simple RC circuit. We want the corner frequency to be as low as possible using 1/2piRC. Since i'm going to use a 100K pot, that will make R2 1M, and so i really don't need that large of a value for C1 to reduce the corner frequency, as R2 is already incredibly high. .22 uF sounds good to me. Ironically RC and RCL circuits are going to be on my final for Physics II, so it's good i understand this.

Now that the pesky C1 is out of the way, lets move onto the gain. From what i understand, gain is how much amplification the sound "gains" as it passes through the amplifier. The gain you choose should depend on your source and the impedence of your headphones. My most likely source is from a game theatre XP 6.1 sound card, which i'm sure is pretty sucky. But i really don't have any experience testing different gains with different sources and headphones, so all i have to go by are your recommendations. I am using the HD600 headphones which have 300 ohm impedence. The default gain that you have setup on your diagram tangent is +9db. Do you think that a gain of 9 is enough, or would 13 be more appropriate for my setup. I probably want to use DIP sockets in conjunction with these resistors just in case i want to change the gain later (any suggestions on what DIP sockets fit best?), let me know what you think.

Now, the Op-Amp. It is a dual channel op-amp, but why do you mount it on a browndog adaptor, i thought this was only for single channel chips so that you could mount 2 of them together onto the board?

As for D2 vs D1, since i am only using wallpower, i will go with D2, but what part number would you use, i don't see one in your list. The reason i asked if you could use D1 in conjunction with D2 is because of the pictures in this link: http://www4.head-fi.org/forums/showt...threadid=20835
It appears as though those two orange blocks are diodes occupying D1 and D2, please correct me if im wrong.


The case i am using is in fact the same pactec plastic case that you see in the link above, which i didn't even think of when considering grounding, so does that mean i won't have to ground the neutrik output jack or the ALPS pot either. I'm thinking no since it is surrounded by plastic.

The resistor part numbers that i am using are as follows:
71-RN55D-F-resistor value here, I'm not sure what the trade off is between 1% and .1% but i'm guessing that 1% is fine.



Now, R10. Still do not fully understand how the value of this part influences the output, other than it helps to bias the op-amp into class A. Tangent, you first recommended 1K to me, and now you are saying 100 ohms, will i really not notice any difference between the two, or should i just throw a 100 in there and call it a day. Same deal for R7. It says that it decreases the "ringing" on the op-amps output. Does it matter if i use a 51 ohm or a 100 ohm resistor in this spot?


I will go with the 24v power supply you suggested. But i am still unsure of what i need to mount in the case. Is the jack for this power supply the "wall wart"? Also, what kind of cable would i use to connect the power supply to the wall wart.

I think that covers everything. I will be using Belden 22awg teflon coated, tinned copper, stranded wire a suggestion from JMT, and i'll probably pick up a 1/2 lb of cardas solder. My old robotics teacher is lending me a really nice soldering station to use with digital temperature control, so that will be very useful.

The last thing that i need to know, is what modifications i will need to make to the case in order to mount the jacks and pot, and what tools i should use to do this.

Thanks

 

[jasong]

The thread you linked to shows a 1N5822 diode on a Version 1 Meta42. It's the black cylinder with a stripe at one end. The diode placements have been moved on V2, look at the the current board layout at Tangent's site.

Yes, socket the resistors to play with the gain. Search for Millmax on the part sites. You need DIP-14's.

The op-amp does not have to be mounted on a browndog if it is a DIP package, like the OPA2132. But if the op-amp is a surface mount (tiny-ass) package like the 8512 or 8620, you need the Browndog. I would get the 8620 on the Browndog, solder a DIP-8 socket on the board, and insert the Browndog assembly into the socket. Watch out for pin 1...

A wall wart is a slang term for a power supply with prongs that plugs into wall outlet. This, then, has a thin cable that comes out the bottom with a plug that connects to your device. The power supply jack is the part that the plug fits into. The jack is what gets installed in your case.

Use what Tangent recommends for R10 in the body of the current source section...

--Jasong

 

[tangent]

Quote:

My most likely source is from a game theatre XP 6.1 sound card, which i'm sure is pretty sucky

That is true, this may well suck for sound quality, but it should be line level, so you don't need to compensate with a high gain like you would for a PDCP or something like that.

What I would do is get several different possible resistors for R2 and do some experimenting. Set the gain too low and you can't get enough volume. Set it too high and the volume knob is touchy and the noise floor is artificially raised.

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It is a dual channel op-amp, but why do you mount it on a browndog adaptor,

So that you can plug it into a socket, so you can remove it if needed. If you want to solder the chip to the board and are therefore confident that you will never have to remove it, go right on ahead -- the board has SO-8 pads for this purpose. Desoldering SO-8 parts without damaging chip or board is tricky.

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i will go with D2, but what part number would you use, i don't see one in your list.

Use a 1N400x.

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It appears as though those two orange blocks are diodes occupying D1 and D2, please correct me if im wrong.

Those are the Wima MKS-2 C4s.

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does that mean i won't have to ground the neutrik output jack or the ALPS pot either.

You still ground the pot to the board's PG pad.

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i'm guessing that 1% is fine.

1% resistors are much cheaper, and if you have a random batch of them, on average you can find a pair matching to within 0.1% with 4.8 draws from the pile. This number comes from a Monte Carlo simulation program I cooked up. In real life, you often get batches of resistors with better matching than the manufacturer specifies. Point being, it's easy to hand-match 1% resistors to 0.1%, just so long as you get enough of them.

As for R7 and R10, I have no easy answers for you. If you want the most perfect resistors for your situation, you'll have to do some measurements and listening tests. All I'm trying to do is give you is a range of reasonable values to try.

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Is the jack for this power supply the "wall wart"?

A "wall wart" is the big black brick that plugs into the wall -- the wart on your wall. More properly called AC-DC power supplies, sometimes loosely called "transformers", also "wall bug", "power supply", "bloody stupid black brick", etc.

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what modifications i will need to make to the case in order to mount the jacks and pot, and what tools i should use to do this.

Go to my site, get up to the main audio level, go into the DIY newbies article, and find the link to Apheared's Project Casing Guide. I think there was also a similar thread here at Head-Fi recently.

 

[Kal525]

Alright, sounds good, i think i'm almost ready to start ordering. I guess i'll just use 100 ohms for R7 and R10. Would you recommend ordering more resistors than i need? And if i do order more than i need, how do i check to see which resistors are best?

Also, with the wallwart you recommended, what jack should i mount in the case for it and what cable do i need to buy to connect the two?

thanks

 

[tangent]

Quote:

Would you recommend ordering more resistors than i need?

Yes, by all means, get at least 10 of every value you need. For values like the 100 ohm, which you need several of, get 20 or 30. This gives you several samples to choose from, and the extras you can put in a bin for experimentation on later projects.

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And if i do order more than i need, how do i check to see which resistors are best?

Use an ohmmeter (one function of a digital multimeter), pull resistors one at a time from the pile, measure, and keep going until you find two that are within 0.1% of each other. It should only take about 4 tries on average to find a suitable pair. If you need a "1K" resistor, it doesn't matter in the META42 if they're actually 1090 ohms or 982 ohms, or whatever. What matters is that both R1s are within 0.1% of each other, and the same for all other left/right pairs.

This is not an essential tweak, but it's cheap and easy, so I highly recommend it.

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Also, with the wallwart you recommended, what jack should i mount in the case for it

Use Radio Shack's heavy duty 5.5mm/2.5mm DC power jack, or the similar unit by DGS, available from Mouser. I should put this in the docs, but I haven't yet.

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what cable do i need to buy to connect the two?

Have you never seen a wall wart in your life? The cable is attached to the brick part.

 

[Kal525]

ok, i think i have all the info i need to start ordering, thanks for the help.........and sorry i've never heard of a "wall wart" before. I didn't want to assume that the cable came on the "brick" and that it would magically fit in any the jack that i buy for it, since i'm pretty sure there is more than one type of jack that you can buy, but i guess everything will work out. thanks again for the help.

 

[andrzejpw]

Quote:

you're probably the kind of guy who should think about jumpering C1

Wait, when you say jumper c1, do you mean just leave it unpopulated? Don't bad things happen when you jumper caps?

 

[Kal525]

how does this look: http://www.mouser.com/index.cfm?hand...hby=PartNumber

and are you certain that the wall wart you recommended to me comes with the cable, because it says nothing about it in the catalog, it even shows the brick with a place where you would plug the DC connector.

 

[Kal525]

So i use the same diode that i would have used in D1. You recommended this earlier, would it be suitable for AC power as well:

Schotky crowbar diode, 40V 3A, DO-201 package

What about these for the DIP-14 sockets, there were a number one, i think the open face is what you have pictured though.

http://www.mouser.com/index.cfm?hand...hby=PartNumber

And for the knob, which one of these do i want:

KNOB CLR GLOSS.50"DIA 6MM SHAFT

or

KNOB CLR GLOSS.75"DIA 6MM SHAFT

these are the kilo ML knobs to be used with the ALPS pot


Edit: ok, this has become too expensive, i'm going with a panasonic EVJ-C20 pot, my question is, what volume know will fit this. There are some listed in the parts list, but i'm not sure if they all fit the panasonic pot. One is ALUMINUM .61"D and the other from digikey says .75" but i can't find any measurement like this for the panasonic pot except for 12 mm. any ideas.

 

[Voodoochile]

Hi there-
Yes, the Kilo knobs will fit the Panasonic pots just fine. I have used the pair on several METAs.

Quote:

Wait, when you say jumper c1, do you mean just leave it unpopulated? Don't bad things happen when you jumper caps?

Perhaps if you jumpered a charged cap. This was meant to indicate jumpering the position, not the cap itself. If unpopulated and unjumpered, the signal will not pass.

In the case of this particular amp, I'd install C1

. You can always cut it out later and jumper the pads.