Feeler: 1084 Eq/Preamp in 1u with extended frequency ranges.

[iampoor1]

Hey guys

Correct me if Im wrong, but it looks like most of the 1084 eq/mic preamp projects have dried up.

So of course, that got me to thinking, why not start an "all in 1" 1u 1084 preamp/eq project,, sort of like Bluzzis EZ1084, but with the preamp built on the board.  8)

I am thinking of these frequency bands:
High pass- Off, 25, 45, 70, 160, 360
Low Band: Off, 20, 35, 60, 110, 220
Mid band (Plus 3 position "Q" switch. 1084, 1084 high Q, 1073 High Q) 270, 360, 500, 700, 1.1khz, 1.6khz, 2.4khz, 3.2khz, 4.8khz, 7.2khz, 10khz
High band: Off, 10k, 12k, 16k, 20k, 30k

What do you think of these frequency ranges? Of course, it would be possible to extend the ranges, but would then need more expensive switches.

Goal is to have all rotary switches and pots PCB mounted, with only the transformers needing external wiring.

Thoughts? If the project was well executed who would be interested, and in how many channels? I am completly open to doing all the design work for this, just dont really want to if only me and 2 other other people are interested.  ;D

 

[Ilya]

Does the 1073 ever had the hi-q option? AFAIK hi-q @1084 is a 1073 q, and normal q on 1084 is wider than on 1073.
Also, I'd argue the 20Hz and 30kHz are not very useful. Besides, at 30kHz the frequency response of the unit starts to fall off.

 

[krabbencutter]

Does the 1073 ever had the hi-q option? AFAIK hi-q @1084 is a 1073 q, and normal q on 1084 is wider than on 1073.
Also, I'd argue the 20Hz and 30kHz are not very useful. Besides, at 30kHz the frequency response of the unit starts to fall off.

From a practical standpoint, it's pretty misleading that they decided to label that switch Hi-Q. Because a) the 1073, which has no separate Q switch, has the highest gain and, therefore highest Q, compared to the 1081/1084 and b) the Hi-Q switch acts more like a gain boost.  So if you crank a 1081/1084 to eleven at 'low Q', you'll get 12dB of gain at a slightly wider Q, compared to a 1073. If you flip the Hi-Q switch, you're simply adding 4dB of available gain, instead of getting the same 12dB boost at a narrower Q.

//addendum
The resistor that is responsible for determining the Q on the presence board (BA211), is the resistor right between the inductor and the BA284 input. It's value is 2k7 on the 1073 and 6k8 on the 1084. The Hi-Q in the 1084 switches in a second 7k5 resistor in parallel, so you end up with a total resistance of ~3.566 ohms. You could replace the 7k5 resistor with a value of 4.480 Ohms (= 4k3+180R in the E24 range) which will give you a total resistance of exactly 2k7  That way you should be able to get the 'low' Q of the 1084 and the full range of the 1073.

Regarding the frequency response: removing the 180p & 470p capacitors in the feedback loop of both BA284 amps, would reduce the HF rolloff (see http://auroraaudio.net/ask-geoff/neve-2/tech-tips). But I don't know, if that would introduce other issues.

 

[Ilya]

From a practical standpoint, it's pretty misleading that they decided to label that switch Hi-Q. Because a) the 1073, which has no separate Q switch, has the highest gain and, therefore highest Q, compared to the 1081/1084 and b) the Hi-Q switch acts more like a gain boost.  So if you crank a 1081/1084 to eleven at 'low Q', you'll get 12dB of gain at a slightly wider Q, compared to a 1073. If you flip the Hi-Q switch, you're simply adding 4dB of available gain, instead of getting the same 12dB boost at a narrower Q.

I disagree. You get different Q, Hi-Q being roughly 2 times higher than Low-Q (although max gain depends on the selected Q indeed).

 

[krabbencutter]

I disagree. You get different Q, Hi-Q being roughly 2 times higher than Low-Q (although max gain depends on the selected Q indeed).

Thanks, could you add a plot at the same magnitude?

 

[Ilya]

Thanks, could you add a plot at the same magnitude?

Something like this?

Yes, it looks like the graphs you posted. I disagree with your description of Hi-Q mode as being "just a gain boost". It is not. You can calculate Q factor from the graphs in my previous post - I've left the cursors at the top and -3dB point and see for yourself if the Q is different and what that difference is. This is not a simple gain boost.
BTW the difference between Hi-Q of 1084 and 1073 is around 0.2 (that is, 1073 Q is 0.2 higher than 1084). So I leave this to the TS to decide if it's worth to devote additional section of the switch to get that special 1073 Q factor.

P. S. Nice work on inductors substitution in your other thread. It was an useful and entertaining reading.

 

[krabbencutter]

You're right, and even though I tried to make clear, that it's more of a gain boost than it is a real "narrow" Q mode, I just noticed, that I failed in the last sentence of my og post

So let's wrap it up
- 1073 has the highest Q of the bunch & a bit more gain
- 1084 in it's normal mode has +/-12dB of gain and has a bit wider Q
- Hi-Q extends the gain range of the 1084  to +/-16dB and the Q is nearly identical to the 1073

 

[JCN1218]

I would certainly be interested. The EQ section in particular has been on my wishlist for a while. I actually have one of bluzzi's boards on my shelf. Was hoping he'd do another run so I'd have a stereo pair.

My own personal suggestion would be to just do the EQ section. Correct me if I'm wrong, but the EZ 1290 is still available through the DIYRE shop and through Martin himself last time I checked.

 

[alexc]

+1 for interest here!

I've only used 'chineve' 1081, and spent a lot of time under the hood ...  same entry-level transformers but I upgraded with shielded  Carhill inductors.

Even so, they are the best thing ever.

Not just the mythos but the basic functionality is very satisfying.  Two filters and 4 eq bands.

Just wonderful done in proper magnetics and machined rotary switches 

SO - similar curves and facilities in an opamp outfit is AOK in my book!

Apart from my humble-but-accurate 1081 mod-chinas, I love the all-opamp Neuman eq and very much the old Neotek 4 band sweep'd eq.

IF you can do it, go for it! An affordable DIY player in this niche is always needed.