http://www.spotfire.com.au/blog/ The latest Blog feeds from www.spotfire.com.au en-us Sat, 18 Apr 2026 08:35:48 +1000 Rocketspark http://www.spotfire.com.au/blog/post/11450/a-class-d-hybrid-amplifier-has-phil-gone-bonkas/ Earlier this year I was going around in circles experimenting with a small all-tube Class-AB power amplifier for people who wanted more power than the SE5 can deliver, but still prefered a tube based design.For safety reasons, and convenience, I wanted to keep the external AC power supply, and I wanted to make the amp an identical size to the very successful, but small, PR5 vinyl preamplifier.It didn't take long to realise this was a hiding to nowhere, and the best I could do with two 6BL8 tubes... Sun, 31 Aug 2025 14:34:42 +1000 http://www.spotfire.com.au/blog/#post11450 http://www.spotfire.com.au/blog/post/11348/first-photo--new-30-watt-tube-amplifier/ Here is a photo of the prototype SPOTFIRE hybrid stereo amplifier that started life in the rats-nest on the bench a couple of posts ago. It's in the final stages of design and needs some PCB changes and documentation, but it's an absolute hit performer. Here it is undergoing some two-tone intermodulation testsPower is 35 Watts per channel with both channels driven to full power, and it's flat (+/- 1dB) from 12Hz to 50kHz. The circuit has been tailored to give a nice amount of second harmonic dis... Mon, 30 Jun 2025 21:54:08 +1000 http://www.spotfire.com.au/blog/#post11348 http://www.spotfire.com.au/blog/post/11347/redroo-is-now-spotfire/ For some time I've been searching my mind, unsuccessfully, for a new name for RedRoo tube amplifiers. Suddenly, out of nowhere, SpotFire came to mind, and to my surprise the business name, the trademark for that Class of goods, and the website domain were all available. I like it because the name "Spotfire" could be a compelling and meaningful brand name for a small company making tube audio amplifiers that blend old-school analog warmth with modern technology.According to ChatGPT:"Fire" ev... Sat, 28 Jun 2025 17:39:28 +1000 http://www.spotfire.com.au/blog/#post11347 http://www.spotfire.com.au/blog/post/10673/a-new-years-rats-nest/ Electronics designers use many different prototyping methods. These days, it's very likely that a new design will never actually be prototyped but rather simulated using programs such as SPICE.For very simple circuits like tube amplifiers, I like the rats-nest method. It isn't pretty, but it's fast and effective. It is also way more reliable with tube gear as SPICE models of tubes do not simulate all their characteristics, especially those in overload conditions. That's important because Low-pow... Wed, 01 Jan 2025 10:26:13 +1100 http://www.spotfire.com.au/blog/#post10673 http://www.spotfire.com.au/blog/post/10085/guitar-amplifier-now-available-for-order/ The Red Roo Guitar Amplifier is now available for advanced purchase.Like all Red Roo amplifiers, it uses only top-quality components and is easy to build by anyone with a little electronic construction experience. It's a stand-alone "lunchbox" type amplifier so you will need to use a separate guitar speaker.This is a fully featured progressive-distortion design with a Fender-type tone stack. Presence and Depth controls in the negative feedback path allow you to further tailor the sound and there... Mon, 25 Sep 2023 11:07:25 +1000 http://www.spotfire.com.au/blog/#post10085 http://www.spotfire.com.au/blog/post/10065/se5--noise-and-distortion-sweeps/ I've been playing around with the ARTA audio measurement software from https://artalabs.hr/ Although intended primarily for loudspeaker measurements, it does a very good job for tube amplifiers as well. I use this DAC https://focusrite.com/clarett-plusHere are some spectrum measurements of the SE5 amplifier (UL mode, Standard Transformer, Feedback ON, 3Watts RMS output).First pic: Noise floor with SE5 on but no Bluetooth module installed. There is a small peak at 50Hz but noise is... Sat, 02 Sep 2023 11:10:47 +1000 http://www.spotfire.com.au/blog/#post10065 http://www.spotfire.com.au/blog/post/9980/more-power/ People have asked us how far can they push their SE5 amplifier, power-wise. As it turns out, quite a long way. By connecting a full-wave voltage-doubler circuit to the AC output from the power supply, it's possible to achieve much higher HT voltages. Using an HT voltage of 360V, just below the maximum DC voltage rating of the capacitors (400V), and KT88 tubes, we were able to get about 8.5 Watts RMS sine-wave output @ 5% distortion - 10 Watts when pushed.The power supply has jumpers th... Thu, 15 Jun 2023 17:47:07 +1000 http://www.spotfire.com.au/blog/#post9980 http://www.spotfire.com.au/blog/post/9959/using-bookshelf-speakers/ Here at RedRoo, we receive a lot of questions from people wondering how the SE5 amplifier will work with their small bookshelf speakers. That sounds like an easy question to generally answer YES, but, like anything in electronics, it depends on several factors:Speaker efficiency: The sensitivity or efficiency rating of the speaker is an important factor. Speakers with higher sensitivity ratings require less power to produce a given volume level.Desired volume levels: Consider the volume levels y... Sat, 27 May 2023 13:50:54 +1000 http://www.spotfire.com.au/blog/#post9959 http://www.spotfire.com.au/blog/post/9499/redroo-pr5-vinyl-preamp-now-available/ The RedRoo Vinyl Preamp is now available, both as a full kitset or an information pack with PC boards for people who want to roll their own.I've had it playing for the last few months and it sounds great. This is a high-quality kitset which will rival preamplifiers many times the cost. Its a great partner for the SE5 power amplifier.As far as moving coil cartridge inputs go; I'm still waiting for two Moving Coil transformers, so I haven't listened to that option yet, but hopefully soon. I t... Wed, 21 Sep 2022 14:42:36 +1000 http://www.spotfire.com.au/blog/#post9499 http://www.spotfire.com.au/blog/post/9325/new-redroo-guitar-amplifier/ Here is a photo of the new RedRoo Guitar Amplifier, without its protective cover. This is a top-shelf guitar amplifier with all the controls that a musician needs to create their own unique sound. The kitset is in the final stages and will be available soon.  For most of this year it has been trialled by professional guitarists and is receiving glowing reports., (which is good because I can't play a note!). Check out the Guitar amplifier page here.... Thu, 30 Jun 2022 13:54:52 +1000 http://www.spotfire.com.au/blog/#post9325