Feeler: EZ Tube Lunch Box

[G-Sun]

Comp link:

Ideally, but as yet I do not have a scheme known to work.

Ok, thanks for the info!
Hopefully you'll sort out something later

 

[G-Sun]

The ideal answer would be to find a way to change the second amp gain by 10dB using a switch independently of the make up gain for the EQ. You normally set up the EQ by setting all the controls to flat and with the EQ IN you trim the seconf amp gain for overall unity gain from EQ IN to the OUT of the second amp. Then you switch the EQ out (and the pad IN) and adjust the pad for unity gain. The thing is, you don't know exactly what gain you have set for the second stage. It does not matter - all that matters is it equals the insertion loss of the EQ. To increase this gain by 10dB, the simplest thing is to ad d another resistor in parallel with the second stage preset gain pot via a switch - but you don't know what value to set this to. So....... one way to do this would be to switch in another preset pot and adjust this to increase the gain by 10dB. That way you always get the best noise possible. I normally recommend a 10K pot for the EQ gain setting preset because the mid-way point is 5K and this gives close to 20dB gain. If we make the new preset 5K then at its mid point it should add about 6dB of gain and you can tweak it until you get 10 more.

Thanks!
I don't quite understand this yet.
And both eq gain adjust (+-pot)
or fixed gain switch
seems not so important.
(at least if the extra gain is just adding noise)
So I'll just wait a little to see if you implement any recommendations and revisions regarding this.

 

[ruffrecords]

Thanks!
I don't quite understand this yet.
And both eq gain adjust (+-pot)
or fixed gain switch
seems not so important.
(at least if the extra gain is just adding noise)
So I'll just wait a little to see if you implement any recommendations and revisions regarding this.

You need to understand a bit more about gain and noise. Usually, the overall noise performance of a mic pre is determined by the first amplifier stage. It can usually be specified by an equivalent noise level at the input called EIN (equivalent input noise). This is the parameter most mic pre manufacturers like to quote. Now, there is no such thing as a noise free amplifier but many come pretty close.  EIN is usually specified with a 150 ohm resistor connected across the input since this is the typical source resistance of microphones.. The 150 ohm resistor itself generates noise due simply to its temperature and at room temperature in a 20KHz bandwidth, the noise from a 150 ohm resistor is about -131dBu.

So, suppose we have a perfect mic pre with 80dB of gain connected to a ribbon mic with a source impedance of 150 ohms. Just like the signal, the input noise will be amplified by 80dB so at the output it will be -131 + 80 = -51dBu. It can never be any better than this - it is just physics and there is no getting away form it. Of course, most mic pres are not perfect but the best will only add a dB of so of noise so they come pretty close to perfection.

There are several ways of measuring EIN - you can use an average detector, an rms detector or a quasi peak detector. You can weight the frequency spectrum - A weighting is common but there is also an IEC standard weighting for noise measurements (the idea of both of these is to mirror the frequency response of the ear). As you might expect, the different methods and weightings give different values for EIN. There can be as much as 8dB difference between the different measurements of the same mic pre. The average detector and A weighting tend to give the lowest figures so I bet you can guess which ones the marketing guys like to use. Sometimes these even give figures better than the theoretical noise of the resistor!! - just look at the figures claimed for the THAT mic pre chips.

My mic pres are not perfect and I use the IEC standard weighting and a quasi peak detector to measure EIN which tends to give the worst results of all. My mic pres generally measure better than -120dBu using this method. Using A weighting and an average detector they could be as low as -128dBu. Certainly, people who have used ribbon mics with them have said how quiet they are.

The bottom line is that if you need 80dB of gain, your output noise is going to be somewhere between -40dBu and -50dBu. Assuming the first stage is pretty good, then the noise from later stages will make no difference - they just amplify the signal and the already existing noise.

Cheers

Ian

 

[G-Sun]

Thanks a lot Ian!
That's a fine lecture

Does the Eurcard and Classic differ regarding noise when used as a two stage hi-gain preamp?

On the practical side:
I guess +10 to +40db are my typical needs,
+55 for my RE20 occasionally (I don't yet own a ribbon).
And under -70db noise is ok, target is maybe better than -80db (as it would be then with +40db gain).
With e.g 10db compression that's then down to -70db noise. ok.
And no need to utilize the second stage for preamp.

 

[ruffrecords]

Thanks a lot Ian!
That's a fine lecture

Does the Eurcard and Classic differ regarding noise when used as a two stage hi-gain preamp?

Not really, they measure almost exactly the same.

On the practical side:
I guess +10 to +40db are my typical needs,
+55 for my RE20 occasionally (I don't yet own a ribbon).
And under -70db noise is ok, target is maybe better than -80db (as it would be then with +40db gain).

The mic pre has a stepped gain so the noise varies as the gain varies and the maximum gain is 60dB which seems to be more than you need right now. At 60dB gain the noise wil be about -60dBu. reducing the gain will reduce the noise until it reaches about -80 when it stays the same. Should still be about -80 with 40dB gain as you said.

With e.g 10db compression that's then down to -70db noise. ok.

Depends on how much gain make up you use.

And no need to utilize the second stage for preamp.

No, it is meant to be used for EQ make up.

Cheers

Ian

 

[G-Sun]

Thanks!

At 60dB gain the noise wil be about -60dBu. reducing the gain will reduce the noise until it reaches about -80 when it stays the same. Should still be about -80 with 40dB gain as you said.

Ah, good to know

With e.g 10db compression that's then down to -70db noise. ok.

Depends on how much gain make up you use.

True

 

[rmaier]

PSU is almost done and I'm looking ahead to the multi cable and connector to the preamp chassis (using the same TE system described in post 264).  What is the general consensus on minimum/maximum distance between the PSU and preamps?

Ralph

 

[ruffrecords]

PSU is almost done and I'm looking ahead to the multi cable and connector to the preamp chassis (using the same TE system described in post 264).  What is the general consensus on minimum/maximum distance between the PSU and preamps?

Ralph

I generally try to limit it to three feet or one metre. I was recently asked for six feet and had to say it was untested at that length. Not to say it would not work but just that I don't know.

Cheers

ian

 

[anjing]

PSU is almost done and I'm looking ahead to the multi cable and connector to the preamp chassis (using the same TE system described in post 264).  What is the general consensus on minimum/maximum distance between the PSU and preamps?

Ralph

I generally try to limit it to three feet or one metre. I was recently asked for six feet and had to say it was untested at that length. Not to say it would not work but just that I don't know.

Cheers

ian

I'm away from home now so i can't confirm the exact length, but the connection from power supply to eztube mixer is more than two meters and working fine, i'm using the cable that Ian recommended that i got from Newark.  I'm feeding 8 modules for now.

Regards,

Pierre

 

[rmaier]

Thanks, gents, valuable info.  I guess my main concern is having enough distance between the units to minimize any potential hum issues. I can go as far as about three meters if needed. If, on the other hand, I can keep them in the same portable 6U rack case, that would be a bit more convenient. They'll see most of their use for location work.

 

[ruffrecords]

I'm away from home now so i can't confirm the exact length, but the connection from power supply to eztube mixer is more than two meters and working fine, i'm using the cable that Ian recommended that i got from Newark.  I'm feeding 8 modules for now.

Regards,

Pierre

Thanks for that Pierre. It is good to have other people on the team!!

Cheers

Ian

 

[ruffrecords]

Thanks, gents, valuable info.  I guess my main concern is having enough distance between the units to minimize any potential hum issues. I can go as far as about three meters if needed. If, on the other hand, I can keep them in the same portable 6U rack case, that would be a bit more convenient. They'll see most of their use for location work.

Now the design is more mature and I have played around with it a lot, I am a lot less concerned about hum problems than I was when I started the design. I have even tried ac heaters and had no hum problems.

There is still potential for hum to be induced in mic pre transformers from mains transformers so keeping them apart is a good thing and you also need to make sure they are both well screened. This is usually easy to do with an external power supply that is built into a metal box but it is less easy with the sub-rack that the modules plug into. Most sub-rack kits you buy are little more than two sheets of metal separated by thin extrusions so there is a lot of area that has no screening at all. The front panels will screen the front, and the rear panels will sort out the back. It is the top and bottom that are  not so easily handled. The sub-rack kits I get from SRS include a solid  bottom panel and a perforated top one (to let the heat out). I am sure there are other ways of screening the top and bottom of a sub-rack. At the moment I have a prototype Lunch box with a uilt in power supply so the toriod is very close to the modules. I had Classic channel in there with 70dB of gain and I could not detect hum.

On balance I think that one metre will be plenty.

Cheers

Ian

 

[rmaier]

I've just started sourcing parts for a build that will include 4 Classic preamp boards and separate PSU. I could use a bit of advice selecting toroids (always the most arcane part of any build for me, I have to confess). I would have liked to use the same one from antek as I used in my Poshman EQ, but it's out of stock:

http://www.antekinc.com/as-05t240-50va-240v-transformer/

Can I use this instead? http://www.antekinc.com/as-1t230-100va-230v-transformer/

Thanks!
Ralph

Before I hook this up, I could use some advice. Since this has 2x230 secondaries, can I leave one pair unused or would it be better to use both wired in parallel? Thanks in advance.
Ralph

 

[ruffrecords]

I've just started sourcing parts for a build that will include 4 Classic preamp boards and separate PSU. I could use a bit of advice selecting toroids (always the most arcane part of any build for me, I have to confess). I would have liked to use the same one from antek as I used in my Poshman EQ, but it's out of stock:

http://www.antekinc.com/as-05t240-50va-240v-transformer/

Can I use this instead? http://www.antekinc.com/as-1t230-100va-230v-transformer/

Thanks!
Ralph

Before I hook this up, I could use some advice. Since this has 2x230 secondaries, can I leave one pair unused or would it be better to use both wired in parallel? Thanks in advance.
Ralph

I would wire the two secondaries in parallel. You don't need the current but it will improve the regulation. The transformer is a bit of an overkill but it will work fine.

Cheers

Ian

 

[rmaier]

Almost there... What size fuse should I use?
Thanks!
Ralph

 

[ruffrecords]

Almost there... What size fuse should I use?
Thanks!
Ralph

How many amps will it power?

Cheers

Ian

 

[rmaier]

Sorry! Four.
Ralph

 

[ruffrecords]

Sorry! Four.
Ralph

If I remember correctly you have four Classic boards so that is:

80mA HT at 300V = 24 watts
1.2A heaters at 2.6V = 15watts
Total 38 watts

Assume the power supply is 50% efficient which means the input power is about 80 watts. On a 115V supply that is 695mA input current so I would say a 1 amp fuse will be OK.

Cheers

Ian

 

[rmaier]

Well I fired up the PSU today and it passed the smoke test (whew).
I'm getting the following readings:

HT: 331VDC
HTR: 11.62VDC
Phantom: 47.9VDC

Ian, you mentioned elsewhere that your designs aren't particularly sensitive to HT voltages as long as they were roughly between 275-325VDC. My HT is just outside of that... should this be addressed and, if so, how?
Thanks!

Ralph

 

[ruffrecords]

Well I fired up the PSU today and it passed the smoke test (whew).
I'm getting the following readings:

HT: 331VDC
HTR: 11.62VDC
Phantom: 47.9VDC

Ian, you mentioned elsewhere that your designs aren't particularly sensitive to HT voltages as long as they were roughly between 275-325VDC. My HT is just outside of that... should this be addressed and, if so, how?
Thanks!

Ralph

I assume these figures are with just the power supply alone and not connected to any amps? If so then the HT value is fine. It will get lower as you add load to it. The 48V is fine. The heater voltage is a little low.

Cheers

Ian