diyAudio - abraxalito
Go Back   Home > Forums > Blogs

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving

Discrete transistor based CFB I/V stage

Posted 25th September 2015 at 01:56 PM by abraxalito
Updated 26th September 2015 at 01:35 AM by abraxalito

Since I figured out the reason for needing all those caps in my earlier DAC designs was all brought on by using passive I/V, I'm now a total convert of active I/V in order to do away with the sheer bulk.

Having tried single transistor I/V and loved it, I found there was still some improvement to be gained by biassing the common-base transistor with additional current sources to reduce its input impedance. Since getting down to the region of 1ohm would require some 25mA of bias which isn't well suited to portable applications I decided to have a go at using feedback to obtain the impedance I'm seeking.

I'm not using an off-the-peg CFB amp because they still turn out to be fairly power supply quality susceptible (subjectively speaking) so here's a design I hope that greatly reduces the supply impedance requirements so that it can be used in a portable player.

The picture shows the second prototype I/V stage, coupled to a 6th order Chebyshev anti-imaging...
Attached Thumbnails
Click image for larger version

Name:	dac_be.jpg
Views:	35
Size:	145.4 KB
ID:	1724   Click image for larger version

Name:	CFBIV.jpg
Views:	62
Size:	66.2 KB
ID:	1725  
abraxalito's Avatar
diyAudio Member
Views 157 Comments 0 abraxalito is offline

Designing high order active filters

Posted 20th September 2015 at 11:54 PM by abraxalito
Updated 25th September 2015 at 01:20 AM by abraxalito

I've been getting a lot of use out of Simon Bramble's webpage for designing active filters recently - Its a great resource.

Right down at the bottom of the page the last filter he shows the schematic of is a 9th order Chebyshev, 1dB ripple, with a corner frequency of 1kHz. A textbook frequency response plot is obtained using LTC6241s. I latched on to this and tried changing the corner frequency to 18kHz, wondering if I could use such a design for an anti-imaging filter for my DACs. So I divided all the capacitor values by 18 and ran the sim. Disaster! The frequency response I obtained is below - a 7dB spike at 17kHz.

The problem seems to be inadequate Q - high order filters are composed of sections which increase in Q (more positive feedback) and the chosen opamps aren't fast enough (18MHz GBW). I went to a faster opamp for the highest Q stage which brought about some improvement...
Attached Thumbnails
Click image for larger version

Name:	bramble18kchebyshev.jpg
Views:	21
Size:	31.5 KB
ID:	1719   Click image for larger version

Name:	LTC6244-17.4k.jpg
Views:	16
Size:	47.6 KB
ID:	1721   Click image for larger version

Name:	NOSdroop.jpg
Views:	29
Size:	44.3 KB
ID:	1722   Click image for larger version

Name:	NOSdroopFR.jpg
Views:	13
Size:	34.9 KB
ID:	1723  
abraxalito's Avatar
diyAudio Member
Views 253 Comments 7 abraxalito is offline

Taobao headphone amp is an FX box

Posted 14th September 2015 at 01:19 PM by abraxalito

I bought this amp because the case attracted me - no pics on the Taobao page were giving away anything about the insides, quite unusual. After receiving it I couldn't resist having a quick listen and it turns out its a hardwired tone control with bass and treble turned up to the max, about +16dB wrt 1.3kHz. What a surprise! - no matter as I was going to strip out the innards anyway to use as a test platform for my amp.
Attached Thumbnails
Click image for larger version

Name:	fxb.jpg
Views:	26
Size:	59.5 KB
ID:	1716  
abraxalito's Avatar
diyAudio Member
Posted in Uncategorized
Views 211 Comments 0 abraxalito is offline

Can you identify this opamp from its simplified schematic and PSRR plot?

Posted 10th September 2015 at 01:28 AM by abraxalito
Updated 21st September 2015 at 12:23 AM by abraxalito

In my search for opamps with better real-world PSRR behaviour, I came across this beauty. For now I'll just post up its simplified schematic and PSRR plot - if any of you know of it please put your deduction in the comments. I may add more clues later if nobody nails it early on.

The reason I find this part interesting is its cascoded output stage - I believe this is what leads to the 'hump' in the positive rail PSRR. I've never seen that behaviour on any other device.

Well over a hundred views now and not one single stab at the answer. Its the industrial-strength version of the now obsolete LM308, a Bob Widlar special with super-beta input stage. The output stage cascode I take it isn't primarily to improve the PSRR rather its due to the high maximum operating voltage (72V). TI does still have the DS on its website though it doesn't put in an appearance in parametric search as its been obsolete for a while. Even more interesting is its decompensated variant which...
Attached Thumbnails
Click image for larger version

Name:	exhibitA.jpg
Views:	35
Size:	57.2 KB
ID:	1713   Click image for larger version

Name:	exhibitC.jpg
Views:	23
Size:	25.9 KB
ID:	1715  
abraxalito's Avatar
diyAudio Member
Posted in Uncategorized
Views 239 Comments 0 abraxalito is offline

Headphone amps designed for PSRR

Posted 28th August 2015 at 04:38 AM by abraxalito
Updated 6th September 2015 at 10:57 PM by abraxalito

Since acquiring and modding my Taobao headphone amp I've been enamoured of creating a much more portable headphone solution to deliver aural nirvana but on the move. Whereas transformers are a very practical solution for a desktop amp, steel and copper is not only bulky but also jolly heavy and hence a no-no for anything pocket-sized.

How else to get the dynamics I'm seeking though? For these amps I'm toying with different solutions to getting better PSRR, particularly in the all-important bass region which tends to suffer in commercial portable amps. The OPS (output stage) is what needs most attention in any classAB amp - the signal stages can all be classA but for efficiency (and hence battery life) the output stage can't be conducting all the time.

A fully discrete output stage where the output devices are cascoded looks to be one solution but initially I'm looking for a simpler, more cost-effective solution with lower overhead on the supplies if possible....
Attached Thumbnails
Click image for larger version

Name:	psrrprots.jpg
Views:	29
Size:	197.3 KB
ID:	1693   Click image for larger version

Name:	P1010018.JPG
Views:	25
Size:	676.8 KB
ID:	1695   Click image for larger version

Name:	3genHA.jpg
Views:	26
Size:	199.8 KB
ID:	1700   Click image for larger version

Name:	3agenHA.jpg
Views:	23
Size:	350.7 KB
ID:	1701  
Attached Images
File Type: jpg PRv1.jpg (154.6 KB, 223 views)
abraxalito's Avatar
diyAudio Member
Posted in Uncategorized
Views 393 Comments 0 abraxalito is offline

Another incredibly cheap Taobao toy

Posted 19th July 2015 at 10:00 AM by abraxalito
Updated 10th August 2015 at 10:44 AM by abraxalito (Listening report added; chip pinout added)

When browsing Taobao I can't resist the temptation to try the very cheap stuff. This little TFcard player really takes the biscuit for the lowest price of a digital audio source - 9.9rmb. So I ordered up three, I might just order more soon.

First powering up I tried some FLAC files, these don't work but wav and mp3 work fine and it even finds files below the root directory. Power supply is nominally 12V but its using an EF fed by a zener diode, not an IC reg so almost anything from 6V will probably work. I haven't managed to find the technical blurb for the chip so far so a little reverse engineering was needed. It incorporates a 3.3V regulator to power the IR receiver and the TFcard - I have fitted an additional regulator for the first mod because I wondered if the low-level noise I was hearing was due to interference from the flash card's power draw. Turns out no as when playing .wav files this noise disappears.

Incidentally the 9.9rmb cost includes the remote...
Attached Thumbnails
Click image for larger version

Name:	Taobaomp3.png
Views:	183
Size:	354.6 KB
ID:	1676   Click image for larger version

Name:	mp3chipschem.png
Views:	38
Size:	175.2 KB
ID:	1683  
abraxalito's Avatar
diyAudio Member
Views 561 Comments 0 abraxalito is offline

Bargain beaut - headphone amp

Posted 2nd June 2015 at 05:44 AM by abraxalito
Updated 9th July 2015 at 09:43 AM by abraxalito

When I found this on Taobao a few days ago I could not believe the price wasn't a mistake, or that it was just for the case with nothing inside. But it turned out to be real, so I ordered one - it arrived just now so I'm taking it apart before having a listen. The case oozes quality and the volume control feels silky smooth.

Here - 莱曼(lehmann) 耳放 莱曼构架的好声耳放,全铝机箱,一体耳放-淘宝网

When I've had a listen (gotta search for my 1/4" adapter) I'll get into modding.... Incidentally for those on 110V, it does have a mains voltage selector switch. Amazing.

I traced out the circuit (still haven't listened) and found its pretty much as shown on the Taobao page. 78/79 15V regulators feed BD139/140s and a TL072 sits between the volume pot and the discrete OPS. This stage resembles a diamond, but instead of the more familiar current sources to the rails, it has 1500ohm resistors. LTSpicing this arrangement (schematic shown) reveals its...
Attached Thumbnails
Click image for larger version

Name:	bargainbeaut.png
Views:	326
Size:	412.9 KB
ID:	1656   Click image for larger version

Name:	TBOPS.png
Views:	181
Size:	5.1 KB
ID:	1657   Click image for larger version

Name:	TaobaoCCS.png
Views:	216
Size:	25.6 KB
ID:	1658   Click image for larger version

Name:	P1010013.JPG
Views:	104
Size:	666.6 KB
ID:	1670   Click image for larger version

Name:	P1010014.JPG
Views:	226
Size:	695.5 KB
ID:	1671  

abraxalito's Avatar
diyAudio Member
Views 713 Comments 2 abraxalito is offline

Why might a diamond buffer sound better?

Posted 25th May 2015 at 07:00 AM by abraxalito

Here's an interesting post by Charles Hansen (of Ayre fame) about his preference for diamond buffers in amp output stages -

He's saying that diamond buffers sound better but that he has no idea why this would be.

Simulating the PSRR of the diamond vs a traditional EF2 reveals a significant difference - about 6dB better PSRR for the diamond. Could this explain the SQ improvement? Charles designs his amps without GNFB so you'd tend to think that his OPS PSRR is really rather critical.

Since making this discovery I've been on the look-out for opamps with diamond buffer OPSs - OP260 was one I found (courtesy of Esperado) but there are a few from Linear Technology which I've ordered up a few of. In particular, LT1886 and LT1723 look very interesting as potential amp/buffer stages in DACs. LT is fairly unique in that they publish a simplified internal schematic in all the DSs I've looked at...
abraxalito's Avatar
diyAudio Member
Views 701 Comments 2 abraxalito is offline

What's the PSRR of an emitter follower?

Posted 16th April 2015 at 01:28 AM by abraxalito

I asked this question of Yahoo and didn't find anything much - a few people mentioning the PSRR of their C-multipliers but no simulation results and no algebra. So I fired up LTspice for myself to take a look. The transistor models are the usual Gummel-Poon ones LTspice provides (one up from hybrid-pi) which look to be decent enough for this purpose.

I went for two EFs (sometimes called EF2) as that's probably a more practical arrangement in an audio amp. Some designers even prefer EF3 to get much lighter loading on the VAS/TIS. The EF load I made independent of the bias current so I could learn more about biassing. I used AC simulation to have a look at how the PSRR varied over the audio band.

Turns out the PSRR depends on at least three circuit details and one inherent characteristic of the EF transistor used. In no particular order the circuit aspects which matter are the source impedance seen by the base of the EF, secondly the load impedance seen and finally...
Attached Thumbnails
Click image for larger version

Name:	EF-PSRRsim.png
Views:	178
Size:	10.8 KB
ID:	1617  
abraxalito's Avatar
diyAudio Member
Views 796 Comments 6 abraxalito is offline

Are PSRR plots all they're cracked up to be?

Posted 15th April 2015 at 04:19 AM by abraxalito

My experiments last autumn with putting a transformer between a chipamp and a drive unit left me with one nagging question. Why was the sound so much clearer with the trafo than without, given that the trafo's voltage ratio was relatively low (1.5:1)? Even Frank was surprised at so much change and suggested some other effect was in play (amp instability). I've not ruled out amp instability but I have made some progress on understanding chipamp PSRR.

The first trafo experiment was done with the TDA7265 - I've since tried trafos on two other chipamps - TDA8947 and LM4766 with similar subjective results - a much clearer sound, more depth to the soundstage and so many tiny details on recordings becoming clearer that the experience has become akin to headphone listening. The trafo ratio has increased - 2.5:1 for the TDA8947 (21V rails, bridged) and 5:1 in the case of the LM4766 (again bridged), running on 62V total supply.

My current listening system is LM4766 with...
Attached Thumbnails
Click image for larger version

Name:	lm4766psrr.png
Views:	133
Size:	62.9 KB
ID:	1616  
abraxalito's Avatar
diyAudio Member
Posted in Power chip amps
Views 671 Comments 9 abraxalito is offline
Hide this!Advertise here!

New To Site? Need Help?

All times are GMT. The time now is 09:03 PM.

vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2015 DragonByte Technologies Ltd.
Copyright 1999-2015 diyAudio

Content Relevant URLs by vBSEO 3.3.2