Category Archives: Guest Contributions

Gary Galo AudioXpress preamp series

We are pleased to note that Gary Galo has recently made a very large Guest Contribution to this website. This is in the form of a series of his notable preamp modification articles, that appeared in AudioXpress. The articles are listed below in the sequence they appeared. Click the individual link for a given article. A ZIPfile with all the articles is available as well, at the end.

The four (+) part series on modifications to the Adcom GFP565 preamp:

Part one, from the 11/03 issue of AudioXpress.

Part two, from the 12/03 issue of AudioXpress.

Part three, from the 01/04 issue of AudioXpress.

Part four, from the 02/04 issue of AudioXpress.

A follow up, from the 12/04 issue of AudioXpress.

Miscellaneous letters and corrections.

Another preamp article, on Gary’s design of a music library preamp, for the Crane School of Music, SUNY Potsdam.

A zipped file with all seven articles (~12megs). 

It is a real pleasure to be able to make this landmark series of preamp articles available to the readers of this website.

Our thanks to Gary for making all of this possible, and for executing some superb examples of true DIY craftsmanship!

Walt Jung




Klaus Noll’s ‘Showcase: A Headphone Amplifier’

In Grayson King’s ‘Valkyrie’ Preamp Guest Contribution post, a related piece was mentioned. This one is Klaus Noll’s Showcase: A Headphone Amplifier, which appeared in AudioXpress in May of 2003. We are pleased to add this work by Klaus as another Guest Contribution.

Klaus Noll’s article is similar to Grayson’s in that it describes a line-stage type preamp with a wideband composite amplifier using the AD744/AD811 pair, but optimized for headphone use. It also differs with regard to the power supply regulators used, which are similar to the Improved  Regulators from Audio Electronics of 2000.

In response to my query on posting his article here as a Guest Contribution, Klaus said:

“Thank you very much for your flattering letter which arrived today a little belated. Of course you can put the article on your website, after all it is your intellectual property which I used to build what I think must be one of the five best headphone amplifiers in the world.  Thank you again and kind regards.”

Well thanks to you Klaus, first for building such a fine preamp/headphone amp, and for sharing the details via AudioXpress. And of course, for offering it now, as a Guest Contribution here.

Both Grayson and Klaus have created worthy DIY audio projects, with full PCB patterns and part details, and it is great to be able to reprise them here.



Grayson King’s ‘Valkyrie’ Preamp

Since I’ve been running this website, I have had numerous emails about past articles. These have increased of late, which is a good thing. One of the more interesting developments have been the “By Request” and the “Guest Contributions” categories. Here’s an excerpt of a recent email one from Vladislav Polur, which falls into both. I have made some minor edits, for clarity:

“I am sending you a link about a review of a preamplifier made based on your article about using video op amps in audio. I could not find the author of the preamplifier. The reason I am interested in this design is because I had a chance to listen to an amplifier based just on signal op amps, within my friend’s system. It was a great experience; it sounded better than any amplifier I had a chance to hear before, tube or solid state.”

I think Vladislav may have given me a bit too much credit for the entire preamp realization, even it was/is in part based on my work appearing within Gary Galo’s POOGE-5 article. That box insert did in fact use video op amps, and it was titled High Performance Audio Stages Using Transimpedance Amplifiers. That preliminary piece sets the stage for the highlight item of this post, as follows below.

The item that matches up best with Vladislav’s listening experience and his cited link was Grayson King’s preamp project. This work was from the 1994 series of The Audio Amateur, and was entitled Valkyrie: A Line-Stage Preamplifier. The preamp was Grayson’s senior project while at Clarkson University in pursuit of his EE degree. It was developed under the tutelage of Gary Galo, and it did indeed make not just a fun university project, but also a great example of a worthy project for others. Grayson did do a fine job with his preamp! After graduation he took a job with Analog Devices in Boston, working in the same department as I did then, applications engineering.

It is a pleasure to help Grayson’s preamp to find some new friends here, and I want thank him for helping to make it all available once more, as a “Guest Contribution”.

A final note for those pursuing line stage performance. Another such preamp was described in Klaus Noll’s Showcase: A Headphone Amp, in AudioXpress, May 2003.

Pat Amer and Jim Boak Dynaco ST150 Mods

In The Audio Amateur issue 2 of 1981, two Dynaco ST150 amp modification articles appeared. One was titled ‘The ST-150-BJ-1 A Boak-Jung Modification of the Dynaco Stereo 150 Amplifier’, and it was authored by Pat Amer. This article was on the general ST150 modification process, and it included both circuit upgrades and a set of power regulation mods. The specific power mods were developed by Jim Boak, who also wrote a related article, titled ‘Power Modifications for the ST-150-BJ-1‘. This second article is definitely Dyna ST150 oriented, but Jim also wrote a more general version, within The Audio Amateur issue 1 of 1980, titled ‘A Family of Power Amplifier Regulated Power Supplies‘.

Not only do the articles describe a useful mod path for the Dyna ST150, but they are still generally applicable within almost any power amp, especially the power mods. Many thanks to Pat and Jim for their help in making these articles available.

We are happy to be able to make these three articles once more available, under the category of Guest Contributions. And, it is also a pleasure to renew contact with both Pat and Jim, after 30-plus years!

Mike Sulzer 1980/1981 Regulator Articles

We are very pleased to be able to have available here in PDF format Mike Sulzer’s classic articles from The Audio Amateur, issues 2/1980 and 1/1981. These two articles are: A High Quality Power Supply Regulator for Operational Amplifier Preamplifiers (published in TAA issue 2/80), and Regulators Revisited (published in TAA issue 1/81). A  ZIPfile package of these two articles is available, for fastest downloads.

These two articles are unquestionably classic mileposts among DIY audiophiles, and essentially started the serious development of quality regulation as an integral part of higher performance audio schemes.

Our sincere thanks go to Guest Contribution author Mike Sulzer for responding to the initial request from Waltsblog reader Nikolaos Baxevanakis, and making these articles available. So, enjoy the articles Nikolaos!

Of course, we hope that many other readers (and re-readers) may also do so.

A Dynamic Range and Clipping Indicator

Back in Audio Amateur issue 1 of 1980 David M. White Jr. published a very worthwhile system tool project, titled A Dynamic Range and Clipping Indicator. With a full instruction set including PCB layout, parts list, and a detailed schematic, this design uses an LED array for display of dynamic signal peaks.

Dave recently upgraded the original hardware, as part of a complete system restoration. He has retired to a new home, and is now busy on the listening room and upgrading his home-built electrostatic arrays along with the associated complex crossover and drivers. As for the Clipping Indicator, he has indicated that all parts are still available, but today’s modern LEDs should provide longer lifetimes than did the old parts. Thanks for sharing a useful design with us once again, Dave!

Morrey’s Super Oscillator

Recently, we had an occasion to review some very old Audio Amateur articles, dating all the way back to issue 4 of 1975. Among them was Morrey’s Super Oscillator, by Walter T. Morrey. This article was a landmark piece in a couple of regards. It provided a comprehensive update for the Heath IG-18 sine wave oscillator, a popular test bench instrument of the period. In it Walt Morrey lays out numerous IG-18 improvements, going far beyond the typical DIY “tweaks”. The replacement op amp topology used for the IG-18 is just as sound today as it was in 1975 (setting device availability as a side-issue). In fact, some of the cascoding features used within individual stages were not only superior then, but also remain so today.

It would be interesting to see someone develop this op amp circuit into a discrete part modern realization, using some of today’s improved complementary SMD active parts. Thanks to Walt Morrey for sharing this classic with us!

(originally posted Feb 2013)