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HP 339A Mods & Upgrades

Posted 29th November 2016 at 01:42 AM by SyncTronX
Updated 27th December 2016 at 02:37 AM by SyncTronX

HP339a
Introduction


The HP339a was a state of the art (SOTA) distortion analyzer in the 1980s.
It is a full featured all-in-one product including a volt meter,
low-distortion oscillator, distortion analyzer, AM detector, and VU meter.

There was an in-depth analysis of low distortion oscillators
on DIYAudio on the following link:


<Low-disortion Audio-range Oscillator>
This was followed by a specific HP339a thread, linked here:

<HP339a>

This is a summary of those threads along with the links, conversations, and
posts, about the research and development that went into the suggested modifications to the venerable HP 339A.

Outline



Why HP 339A?

HP339A Post #78
Well, a stock 339A gives you a wide band ac voltmeter, an oscillator and analyzer. A stock one will do you pretty well as long as you have no interest in reading below...
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Old

Grundig Studio RPC 350 need help

Posted 7th November 2016 at 11:01 AM by Keepsake10

Question for transistor experts.
I recovered the amplifier module from a old Grundig studio RPC 350. This amplifier is a classical concept of the time (around 70) differential input with CCS, driver, darlington complementary finals (BDW 94 / 95) It has DC transistor protection for the output and as well a kind of thermic protection switch on the heater sink (this protection cut entirely the main if heater sink get too hot).
I changed all electrolytic Cs and checked the rest of the parts. The rectifier bridge was replaced with fast recovery rectifiers and both 10.000/30V capacitors where replaced with 18.000F/40V. Measured values as shown by Grundig are ok and the offset can be done within Grundig specifications (the old 1Kohm trimmers will be as well replaced with multi-turn ones to get much easier control).
There is one issue I cant get fixed. The original schematic show the T2004 as BD517. Actually in this place was used a small Motorola in TO92 package...
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Old

Goosebumping headphone DAC-amp

Posted 31st October 2016 at 09:42 AM by abraxalito
Updated 1st December 2016 at 04:23 AM by abraxalito

The Intersil D2 chipset based amp arrived and while it sucked pretty badly on speakers I had a hunch that like the STA333BW boards I've been playing with recently, it would acquit itself admirably on headphones, given the much lower demands on the supply.

The mods I've done to this are fairly straightforward - changing the output filter to work more optimally with headphones (via a 7:1 step-down transformer) and tweaking up the power supply a bit.

To run with cans I went for an output load impedance of 1100ohms. This was determined primarily by the choice of inductor (MSS1210 8.2mH from Coilcraft) which in turn was chosen so that the carrier frequency would be below the SRF of the coil. This choice is fairly arbitrary in that the inductor would still work fine above its SRF just it's starting to look like a capacitor. However it turned out that with 8.2mH the cap value required was fairly simple, 5nF so I stuck two 10nF NP0s in series.

Mods in the...
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Old

Using a digital amp as a DAC

Posted 28th October 2016 at 04:19 AM by abraxalito
Updated 4th November 2016 at 04:03 AM by abraxalito

I've been mighty impressed with the STA333BW board (just 62rmb on Taobao, see an earlier post for the link) driving headphones via a transformer (ferrite cored step down 7:1, I've tried various sizes). So much so that I don't much feel the need to upgrade my headphones beyond my Superluxes (HD668B). The sound I'm getting is immensely satisfying - rich (which I got from my TDA1387-based DACs, but not from my phone with its ES9018K2M, that doesn't qualify as rich in tonality) and dynamic beyond anything I've so far heard. I noticed in a recent thread post by TNT (here - http://www.diyaudio.com/forums/vendors-bazaar/283672-new-audio-op-amp-opa1622-6.html#post4868193) he says he doesn't much like listening on headphones due to the 'in-head' quality of the sound. Yeah I used to get that, transformers pretty much get rid of it giving an out-of-the-head-all-around-holographic kind of experience, along with stupendous dynamics. I like listening to classical piano and that's very demanding...
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When is a clock not a clock?

Posted 29th September 2016 at 05:31 PM by Tam Lin

When it goes through a logic gate.

I heard that riddle in 1978 when I worked as a programmer for a computer startup in Silicon Valley. For some reason, it stuck with me although I didnt appreciate its profundity until I started studying DAC design.
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Old

SAA7220 filter replacement project

Posted 20th September 2016 at 01:24 PM by abraxalito

Some years ago I reverse engineered the coefficients of the Philips SAA7220 filter, just for fun. At that time the microcontroller I implemented the filter on (LPC1114) didn't have a fully implemented I2S interface so I needed additional logic to get the data in (an array of 74HC595s) and I used the SSP to output a serial stream, albeit not reliably as I didn't implement a foolproof method of generating WS. Hence the filter wasn't ready for primetime.

Now though STM has introduced a Cortex M0 MCU which does have the dedicated I2S ports to realize a pukka emulation of the SAA7220. At least that's what I'm hoping - I know the code worked on the LPC1114 but I seem to recall I needed a slight overclock - there is a report that the STM32F072 can be overclocked but I shall need to check this out for myself. One major advantage this replacement will have is going to be lower power consumption - something of the order of 100mW or under.

In order to kick this project off,...
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Old

Taobao shopping for 'digital' amps

Posted 18th September 2016 at 03:45 AM by abraxalito
Updated 30th November 2016 at 02:32 AM by abraxalito

A number of interesting amps have caught my eye on Taobao of late, they (except for the last) are true digital amps in the sense that they're 'power DACs'. The first is using ST's STA333W which is a phenomenally cheap fully-integrated digital amp chip, hence the very low price of this board - https://item.taobao.com/item.htm?spm=a230r.1.14.19.4wjcvg&id=526018865372& ns=1&abbucket=13#detailhttps://item.taobao.com/item.htm?spm=a230r.1.14.19.4wjcvg&id=526018865372& ns=1&abbucket=13#detail USB input, speaker out but needs an external power brick of some sort, up to 20V.

The second one's a boxed product, albeit with an external power supply. Its using Intersil's D2-audio chipset (D2-41051), the first implementation I've noticed on Taobao. It uses discrete MOSFETs in the output stage. The enclosure and controls look to be in the PS Audio territory, a cut above the usual Taobao cheap stuff - https://item.taobao.com/item.htm?spm...cket=13#detail...
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Old

Not Another Gosh-darn EL84 Amplifier!

Posted 17th September 2016 at 07:32 PM by ballpencil

.. or NAGA84 for short.

That familiar feeling.. All tube enthusiasts will have a hard time resisting the urge to build an EL84/6BQ5 push pull amplifier, at least to hear for themselves what makes this tube so well-known (and overpriced).

Lurkers of the tube section most probably have already seen the Baby Huey wiki and thread. With such unique local feedback scheme, it's unsurprising that it garners some avid follower. As amazing as the feedback scheme may seem, there are few things i prefer to do differently on that amp. The most prominent thing would be NOT to use a triode for the input/driver stage as the rp of a triode will change with the signal swing and cause inconsistent feedback ratio. That, and the fact that it doesn't measure really well under simulation (has anyone actually simulated that amp before?), motivated me to do my own version of the EL84 push pull amp.

To remove the triode from the feedback equation, there are two options i can...
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Old

Chinese class D amp modules.

Posted 25th August 2016 at 11:42 AM by Stage Sound

I have imported several class D modules from China and all but one have had RFI problems and seemed to me to unusable and probably illegal in a production item. Far to much RFI on the output and looking at the on board components they have only a single stage output filter comprising of a square grey ceramic choke and one or two mylar caps. Has anyone else found the same thing and is there a mod that is known to work? The modules are all very well made using good quality components but like behaving like a radio transmitter.
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Old

Cyrus Two Amplifier & Power Supply Upgrade

Posted 15th August 2016 at 04:28 AM by erictoucan

Cyrus Two would be the best selling and successful British Amp ever made. Definitely, it is the most important product to bring the market standing in its history.

After almost 40 years, it is still a very good amp around. Definitely, very good value for its second hand price.

Cyrus two phone section consists of super matched pair LM394 which is still the best matched pair today.

Here welcomes any idea to maintain and upgrade the Cyrus Two and its Power Supply to improve its performance to race with today's best amp.

For instance,

1. Replace the aged E-cap
2. Replace of coupling cap for better performance
3. Replace low value and better feedback resistor
with higher quality resistor.
4. Replace the important resistors in RIAA section
the best quality type
etc.

However, a lot of component and design is still very good like the Holden & Fisher...
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