solderdude
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Post by solderdude on Nov 28, 2013 13:38:52 GMT
I'am on the case... ermm schematics.
The idea is to build an amplifier that is dedicated to 1 headphone and corrects that one as flat as possible. It is meant to be portable. The amp could also be build without a filter or something like a mild low boost or cut and or highs boost or cut if so desired.
Process:
Design schematics -> build a proto -> design a PCB -> Have PCB made -> build protos on PCB -> test and measure -> (IF N.O.K. return to disgn a PCB) -> tweak if needed -> create 'manual' -> throw it over the fence.
Now in the 'design schematics phase' in which case it is easy to implement ideas from potential users.
The idea now is to make a dedicated portable amp in a 'Neco portable sized' aluminum enclosure (so only usable with 1 type of headphone) which only has 3.5mm stereo in and 3.5mm (or 6.3mm stereo out), has a power switch and LED.
No gain control, no volume control and must be driven from a headphone out of a player (won't work from line-out) single 9V or dual 9V (depends on the used headphone desired level) non rechargeable batteries. Power consumption should be low and capable of driving all kinds of headphones to at least reasonable levels.
gain should be decided on ordering as it cannot be changed easily as it is integrated in the filter design.
low shelving, high shelving and limited bandwidth/frequency notch filter (gain or reduction). Max 10dB filter action possible.
If potential users would like to comment on things they would like to see on this design or do not like... speak now or be happy with what will be offered at a later stage.
Ideas are welcome.
P.S. The idea of a 'standard amplifier' with 'plug-in' modules/filters has already been suggested and looks quite feasable.
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Dave
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Post by Dave on Nov 28, 2013 16:22:29 GMT
Hi Frans, Not strictly on topic but why do you advise non-rechargeable batteries? Just trying to learn a little . I ask because I have a portable C-Moy amp which currently (sorry, pun not intended ) by 2 x PP9 rechargeable batteries. Dave.
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solderdude
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Post by solderdude on Nov 28, 2013 16:36:00 GMT
Because I would have to install a proper charging circuit + charger as well. I will consider it a possible 'wish'
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Dave
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Post by Dave on Nov 28, 2013 16:53:12 GMT
Ah, I see. Not for audio reasons then, which was the thought behind my query. I will carry on using rechargeable batteries in my C-Moy then . Thanks Frans. BTW not heard from the Audiologist yet but am still hopeful.
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solderdude
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Post by solderdude on Nov 28, 2013 17:38:49 GMT
There is no audio reason to use alkaline or rechargeable but there are other things to consider.
People tend to 'drain' batteries until the unit stops working. This is no problem with non-rechargeables as they simply run out of juice and the amp stops working. As the opamps I plan to use work fine down to 3V this means the amp stops working below that voltage. The thing with rechargeables is that a 9V battery (8.4V) should not be drained below 6V.
IF they are depleted below 6V the capacity and longevity drops MUCH faster than when the voltage stays above 6V. The lower the voltage has been the shorter the battery life and capacity will become.
With a single 9V battery you will probably be warned by distortion in louder passages but when the amp is driven from 2 x 9V batteries (in order to drive headphones >300 Ohm to sufficient levels) the problem is worse because in this case the total battery voltage should not drop below 12V.
As the device itself will work perfectly below that voltage (providing you don't play very loud) and battery life is irreversibly shortened.
A 'warning' LED is possible but means more components.
Also the charging circuit comes into play. Sure... you could keep it simple and use a resistor that charges the batteries with a small current that is low enough NOT to overcharge the batteries and high enough to get manageable charging time (say an overnight charge) but most people would want to charge faster (a few hours at most) and preferably using USB power.
A more complex battery management would have to be used (charge and under-voltage protection) which makes the amp more expensive and complicated.
I will look into simple chips that offer complete battery management but most of these chips are designed to work with voltages from 1.2 to 5V and I need 9V or 18V. Add to that such chips are usually designed to work with (info)Lithium batteries and not with NiMH or NiCd.
Also one would need an adapter with 12V or 24V or a charge pump circuit (as in the Zigis).
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Rabbit
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Post by Rabbit on Nov 28, 2013 17:59:54 GMT
The lack of vol pot kind of gets in the way of purity/quality Frans. If you go from a line out, the signal to noise ratio is as low as possible since the vol pot would attenuate the volume and noise with it. ie, if I wanted to listen at low volume, then I would need to turn the mp3 player down low. This doesn't always reduce the noise levels so the sig/noise ratio is raised. Also, add to that noise, the noise of the fitering system and the sig/noise might be quite high. My own preference is to get as high an input into the amp without distortion creeping in and then use the vol pot to decrease both the volume and the noise. Of course .... you know a T40 driver is the type I'm really interested in, but if there are others it would also be tempting to use a filter to help the HD598 bass as well, so in the end, I think I'd need about twenty amps!!! It's a brilliant idea and is yet another first Frans. Follows in the tradition of the Ember and configurability. (Another first - it amazes me how people play that down) The lack of vol pot might be hard to accept for some.
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solderdude
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Post by solderdude on Nov 28, 2013 22:11:09 GMT
This filteramp won't add noise like the original one does but it also is less flexible.
I have been using a non-volpot amp for years (all my portable amps are volpotless) and with the various DAP's I use(d) over the years I have never had any issues with noise, degraded sound or distortion.
I can imagine it could be problematic with IEM's but in that case the gain will be really low and thus the DAP's noise won't be amplified.
The idea/design now is you buy an amp that has -1x gain (the amp is inverting) and you can open it up and insert or most likely solder in a filter board. So... if the headphone is replaced by another one you can simply replace the filter board (just a few passive components) with another one or use a board with just a different gain, creating a high quality C'Moy.
A volpot cannot be added because of the design (minimal amount of parts) for that reason the design also has to be fed from a headphone out anyway. Fortunately the load the headphone has on the DAP in question is very low and turns all DAP output stages in pure class-A buffers. The distortion / roll-off these output stages have UNDER load is gone and signal quality is MUCH higher.
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Deleted
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Post by Deleted on Nov 28, 2013 23:59:21 GMT
Frans,
How about a circuit that measured voltage and "beeped", every minute or so, to warn the user that the batteries should now be recharged.
Would that be too complicated/expensive?
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solderdude
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Post by solderdude on Nov 29, 2013 6:04:42 GMT
That seems like a better idea than a flashing LED.
I will look into a circuit that disconnects the battery (so it stops draining when left on accidentally) below a certain voltage.
Playing time with alkaline will be around 2x longer though.
There is also size to consider... there are 2 pretty big capacitors in there that fill up much space and the filter components I do not want to be SMD sized. I may have to design it in all SMD anyway which may make room for battery management.
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Rabbit
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Post by Rabbit on Nov 29, 2013 8:55:40 GMT
If batteries will be swapped Frans, that connection is always the one that can put me off amps ..... In some cheaper amps, it's a basic connector from the battery (s) soldered directly to the board. With time it can get a bit iffy and the wire frays or gets stressed. Also, the battery covers can also be a cause of annoyance. Those plastic snap ons give up easily too. Also battery fit and rattles - in some amps, I've seen the batteries hitting components inside due to no consideration of fit and how it's held. Basic stuff, but very noticeable to regular users of them. Just thinking about the physical build and some of the things that I haven't liked. The Zigis is done this way and that's the one thing I don't really like about it. Now, the compartment cover rattles and I'm always nervous of battery changes with the wire being held by a solder joint which could suddenly break one day if I stress it bend it the wrong way!! It also 'feels' cheap and makeshift. Concerning the vol pot, I see what you are saying. Many would use something like this with IPods though, (which I know you don't use) The dock connector out is noticeably better sounding than the vol pot out which is what I was also thinking about. The vol out is warmer and less powerful since eu regs come into force whereas the dock is way more robust. I'm thinking more about higher quality headphones that notice these quirks which is what quite a few would want an amp like this for imo. No point really in using an eq unit with buds. I think most use buds purely for convenience. This is exciting!!!!
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solderdude
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Post by solderdude on Nov 29, 2013 11:03:20 GMT
So rechargeable with automatic switch-off is in the cards.
I don't like to swap out batteries either unless it is really easy/fast to do. This generally means plastic enclosures which I do not want.
Now to decide on the power inlet and used charging voltage(s)
a volpot would mean 1 more opamp which ads to battery drain. Possible but would rather leave it out.
The complaint most heard is about volume controls (durability, accidental increase/decrease of level in pockets e.t.c.)
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Rabbit
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Post by Rabbit on Nov 29, 2013 15:48:31 GMT
This is a cheeky one ..... for people who don't like 'flat' .... bass boost?
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solderdude
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Post by solderdude on Nov 29, 2013 15:56:33 GMT
Bass boost, treble boost, treble reduction or bass reduction all possible but fixed, you can't flip a switch and turn it into a 'flat' amp. Thinking off making the filter parts in a 'DIP' socket construction which makes it very easy to 'change' settings.
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Rabbit
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Post by Rabbit on Nov 29, 2013 16:03:23 GMT
A filter bypass switch so at least other headphones could be used if not under ideal (sonic) conditions?
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solderdude
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Post by solderdude on Nov 29, 2013 16:38:41 GMT
If I were to implement a bypass switch more opamps would be needed which means shorter battery life which is already shorter because of the rechargeable batteries.
Will look into it and see if it is possible to do this in the current config.
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