Racking up my INA217 project... What to do for gain?

[NeveSSL]

Hi all!

I am very pleased with the results of my INA217. I built it straight off of the spec sheet, except I'll be adding some audio Nichicon caps to the output to block any DC offset from going out or any phantom from coming in, just in case I do a really poor job patching at some point. I tried finding the datasheets comparing it to a John Hardy, Millenia, and an Avalon that I did for a class, but I couldn't find them. I will, though. Anyway, its flat and clean with low noise. A poor-man's Millenia, IMHO.

So, I've decided to build 6 more channels (*gulp* I need to do PCBs on these... point-to-point on vero board sucks! ) and rack them up.

Here's my question: The spec sheet calls for something ridiculous like a 1.4k reverse log pot for the gain in series with an 8 ohm resistor. That, I believe, is a bit ridiculous. So I'm debating on whether I should try to find a different, suitable replacement pot (I think someone suggested a 10k reverse log pot with another resistor across the wipers?) for the gain or should I just do the rotary switch route?

I have found some inexpensive rotary switches (total cost about $1.50 each for single pole and 12 positions) that would work. But the problem is tweaking. If I go this route, I would probably do 12 increments of around 5db from 0 to 60dB (which would require higher than 5, but you get the point). As I've experienced from Neve 1272s, this works, but its really nice to tweak the input with a "finer" control, so to speak. Maybe something that would allow me to play all the way around that 5dB range while stepping it up with the rotary switch. At the same time, that seems a bit ridiculous if I can just get a single pot that will do it all.

Also, what do you guys think about adding maybe a 10dB pad to each channel? Whats the most transparent way of doing it without adding too much extra cost or complexity? I've seen some basic pad circuits, but I am a bit concerned about significant degradation of the signal.

Last thing... should I worry about adding a balanced output to this pre? And if so, how? Diff amps? Transformers? Flux capacitors? I don't think it would be a huge deal (probably will have short runs of cable from the pre to the converters), but still may be worth doing now for longevity's sake.

Any help or insight is MUCH appreciated!

Brandon

 

[NeveSSL]

Sweet... just found this thread on balancing the output of the INA217... I actually think I may since it wouldn't cost much. Would a TL072 mess with the sound in any significant way? I'm assuming not, but I rather like my pres the way they sound now.

Also, here is my build thread, FWIW. And also, I did graduate in August due to that little project. Good stuff! Now I'm working on my Masters.

Brandon

 

[mshilarious]

Sweet... just found this thread on balancing the output of the INA217... I actually think I may since it wouldn't cost much. Would a TL072 mess with the sound in any significant way? I'm assuming not, but I rather like my pres the way they sound now.

Use OPA2134, it's quite a bit nicer than TL072. At low gains, that will increase noise on one output though, since it's not as quiet as INA217. Almost no IC is! But if you have at least 12dB of gain, it shouldn't be an issue.

If you are going the rotary switch route, just add a trimpot off the output. You can limit that to a 10dB cut by adding a resistor. I don't really worry about precise gain because it's just not that important; 5dB increments should work fine unless you have to match levels on two mics that are badly matched or something . . . you will need to buffer the output if you use a trimpot after the INA217, but you have two channels of opamp with OPA2134 (or TL072), so make one inverting and one noninverting.

On the output, I would not only use 63V caps, but also clamping diodes to protect the IC from any woes coming its way. That's a good idea on the input too.

 

[Minion]

For the Full 60db gain range of the INA217 you would need a Reverse Log 10K pot ......You could also easilly build a Stepped Switch useing the Values listed in the Datasheet....

You can find inexpensive Alps pots in this Value at Smallbear.com....(remember you have to Wire the pots as Variable resistors as opposed as Voltage dividers)

In My INA217 preamps I used a NE5532 to Ballance the output , I also used a TL072 in the DC Offset controll loop but if you are useing an output cap you don"t need it for the DC Ofset .....

You can also use a 1:1 audio transformer to ballance the output......


Yes makeing projects with Vero or proto board sucks , you should try makeing your own PCB"s , it is actually extremely simple and cheap and makes everything so much easier.....

Cheers

 

[evm1024]

Shameless plug

I would recommend the National Semi LME49860 for your balanced out. This device can handle 44 volts rail to rail and would allow you to get 26 dBu (0 dBFS with a +8 dBu reference) Not that you need to do that.

Plus I'm selling them on eBay right now trying to pay off the bunch that I ordered. (Shameless plug)

As MSH has noted before all instrumentation amp based preamps sound very much the same (ina163, ina217 THAT1512 etc). I've been doing a lot of research into making them sound better and what makes them sound different form each other.

I have to conclude that the "sound" differences between them fall mainly to the tradeoffs and to the passive components. Take a look at the RANE MS-1b schematic to see what they do for a very nice balanced out....

http://www.rane.com/pdf/ms1bsch.pdf

So skip any lessor opamp for the outputs. You need something that is on par with the ina163.

Regards, Ethan

 

[NeveSSL]

Very cool. Thanks for all of the replies, guys!

Would you guys add a phase switch to this pre? I would kind of like to, simply for the fact I plan on building a couple for some friends. Any ideas on how?

Same thing with a pad... any ideas?

I will probably use the OPA2134 for the balancing as they're not terribly expensive ($2.76 at Mouser).

Forgive my ignorance, and I looked at the data sheet, but how do I set it up for unity gain? Would the values in this link be what I would also need for the OPA2134? Its been a while since I went over op amps in class.

Minion: I am actually planning on using your 4-channel PCB that you shared with me in June. Do you have one that has the dual op-amp balanced outs on it as well? Just curious.

Also, is there anywhere other than small bear that does the 10k pots? $2.75 seems a bit expensive to me... but maybe I'm just off base. They are good pots, though.

Thanks for the ... uh... "un-biased suggestion" on the LME49860, evm. I think I'll stick with the OPA2134, though.

Thanks guys for all of your help. It is REALLY appreciated it. I'm hoping I may be able to build a couple for friends and offset the cost of mine.

One last question: What should I do about the power supply in the rack? Should I try to "quarantine" it off on its own? I've got the FiveFish 1 rack unit power supply kit, which comes with a toroidal transformer. Should it be quiet enough on its own as long as my 110V AC lines are as short and twisted as possible? Related to that, do you guys think a 6" deep rack chassis would be deep enough? What are you guys putting yours into?

Thanks much!

Brandon

 

[evm1024]

$2.75

Brandon,

The OPA2134 is a good friend of mine. If you use sockets you can drop in almost any dual opamp (like the LME49860) sometime in the future.

It has been noted that the PS is very important, use the best that you can.

Let's see, I was looking at high quality 1k reverse log pots for $27 each. Wish I could find them for $3....

Regards, Ethan

 

[mshilarious]

Very cool. Thanks for all of the replies, guys!

Would you guys add a phase switch to this pre? I would kind of like to, simply for the fact I plan on building a couple for some friends. Any ideas on how?

I wouldn't, because that's trivial in software. I don't think I've ever pressed that switch on my pres. Anyway, you just need a DPDT switch on the input or output, your choice. Toggle switches are the cheapest.

Same thing with a pad... any ideas?

Do you need it? If you set up for minimum gain of 0dB or +6dB or so, then even a -30dBV/Pa condenser on a 134dBSPL source is +10dBV, well within your headroom.

Anyway, a pad is three resistors. I would not worry about quality, because resistors are unavoidable! If you select proper values, they will add very very little noise, and who cares? You don't use a pad on a quiet source anyway.

I had one guy grill me repeatedly about whether or not my inline pads ($12) used the highest quality components and were completely colorless. Dude, if you're terrified over the prospect of three 1% metal film resistors, just give up recording now!

Also, is there anywhere other than small bear that does the 10k pots? $2.75 seems a bit expensive to me... but maybe I'm just off base. They are good pots, though.

Pots are expensive, especially in small quantities.

One last question: What should I do about the power supply in the rack? Should I try to "quarantine" it off on its own? I've got the FiveFish 1 rack unit power supply kit, which comes with a toroidal transformer. Should it be quiet enough on its own as long as my 110V AC lines are as short and twisted as possible? Related to that, do you guys think a 6" deep rack chassis would be deep enough? What are you guys putting yours into?

Depends on your circuit board, I guess, and how many channels and how many rack spaces . . . You can isolate the transformer inside the case if you have to, but the trace length of the unbalanced signal (from INA to OPA) should be really short and I'd guess you won't have a problem.

 

[NeveSSL]

Ethan: I believe I have a good power supply. I just went ahead and used the one from Five Fish DIY and its been great.

Thanks mshilarious. Quite honestly, the reason I want a pad is more for my converters... I'm using an M-Audio Firewire 1814... max input on the line-inputs is a whopping +2.1dBv... rather lame, but its what I've got for now.

Really, I should just get new converters... so maybe I will.

Thanks again guys!

Brandon

 

[Minion]

Minion: I am actually planning on using your 4-channel PCB that you shared with me in June. Do you have one that has the dual op-amp balanced outs on it as well? Just curious.

Thanks much!

Brandon

I do have a PCB for 4 a Ballanced output with a DC Offset controll loop version but the PCB was one of the first I designed and it is not layed out well and is much larger than it should be , not my best Job but it does work....

There is also a way you can make a Ballanced output without an Opamp or transformer , the technique is called Impedance Ballanced and only takes an extra capacitor and resistor and an optional ferrite bead...

if you look at the output section of this schematic it shows how it is done...

http://www.jensen-transformers.com/as/as017.pdf


Cheers

 

[NeveSSL]

Cool deal. Thanks minion!

Its funny you mention the impedance balanced circuit because I've actually met Bill Whitlock (the Pres of Jensen) and he did a presentation at an AES meeting at MTSU about how balanced lines are balanced more because of impedance than the traditional school of thought... or something along those lines. It was pretty crazy and he went through and thoroughly explained why.

If you get a chance, those white papers on the Jensen website are quite a read.

Brandon